A/B System Updates style partitioning support

Google Pixel XL uses [A/B System Updates](https://source.android.com/devices/tech/ota/ab_updates.html) (or "Seamless Updates"), where there are multiple system, vendor, boot, among other partitions, using a _a or _b suffix on the name. The purpose of these is that a running OS would install an update by flashing the non-active slot, before trying to boot into it, and users would always have a bootable slot in the event that an update fails.

Due to the naming of partitions on systems with A/B System Updates, the [hard coded names](https://github.com/dlenski/tetherback/blob/ac5400bf9a45294988520d5d92c81165939a19ee/tetherback/tetherback.py#L188) will not work.

```
$ adb -d shell 'cat /proc/cmdline' | tr '[:space:]' '\n' | grep '^androidboot.slot_suffix='
androidboot.slot_suffix=_b

$ adb -d shell 'ls -l /dev/block/bootdevice/by-name'
__bionic_open_tzdata: couldn't find any tzdata when looking for GMT!
lrwxrwxrwx 1 root root 16 Jan 3 1970 aboot_a -> /dev/block/sda17
lrwxrwxrwx 1 root root 16 Jan 3 1970 aboot_b -> /dev/block/sda18
lrwxrwxrwx 1 root root 16 Jan 3 1970 apdp_a -> /dev/block/sda29
lrwxrwxrwx 1 root root 16 Jan 3 1970 apdp_b -> /dev/block/sda30
lrwxrwxrwx 1 root root 15 Jan 3 1970 board_info -> /dev/block/sdf1
lrwxrwxrwx 1 root root 16 Jan 3 1970 boot_a -> /dev/block/sda19
lrwxrwxrwx 1 root root 16 Jan 3 1970 boot_b -> /dev/block/sda20
lrwxrwxrwx 1 root root 15 Jan 3 1970 bootlocker_a -> /dev/block/sda1
lrwxrwxrwx 1 root root 15 Jan 3 1970 bootlocker_b -> /dev/block/sda2
lrwxrwxrwx 1 root root 16 Jan 3 1970 cdt -> /dev/block/sdd12
lrwxrwxrwx 1 root root 16 Jan 3 1970 cmnlib32_a -> /dev/block/sda13
lrwxrwxrwx 1 root root 16 Jan 3 1970 cmnlib32_b -> /dev/block/sda14
lrwxrwxrwx 1 root root 16 Jan 3 1970 cmnlib64_a -> /dev/block/sda15
lrwxrwxrwx 1 root root 16 Jan 3 1970 cmnlib64_b -> /dev/block/sda16
lrwxrwxrwx 1 root root 16 Jan 3 1970 ddr -> /dev/block/sdd11
lrwxrwxrwx 1 root root 16 Jan 3 1970 devcfg_a -> /dev/block/sda23
lrwxrwxrwx 1 root root 16 Jan 3 1970 devcfg_b -> /dev/block/sda24
lrwxrwxrwx 1 root root 15 Jan 3 1970 devinfo -> /dev/block/sdf4
lrwxrwxrwx 1 root root 15 Jan 3 1970 devinfobak -> /dev/block/sdd5
lrwxrwxrwx 1 root root 15 Jan 3 1970 dip -> /dev/block/sdd7
lrwxrwxrwx 1 root root 15 Jan 3 1970 dpo -> /dev/block/sdd6
lrwxrwxrwx 1 root root 15 Jan 3 1970 frp -> /dev/block/sde1
lrwxrwxrwx 1 root root 15 Jan 3 1970 fsc -> /dev/block/sdd8
lrwxrwxrwx 1 root root 15 Jan 3 1970 fsg -> /dev/block/sdf3
lrwxrwxrwx 1 root root 16 Jan 3 1970 hosd_a -> /dev/block/sda21
lrwxrwxrwx 1 root root 16 Jan 3 1970 hosd_b -> /dev/block/sda22
lrwxrwxrwx 1 root root 16 Jan 3 1970 hyp_a -> /dev/block/sda11
lrwxrwxrwx 1 root root 16 Jan 3 1970 hyp_b -> /dev/block/sda12
lrwxrwxrwx 1 root root 15 Jan 3 1970 keymaster_a -> /dev/block/sda3
lrwxrwxrwx 1 root root 15 Jan 3 1970 keymaster_b -> /dev/block/sda4
lrwxrwxrwx 1 root root 15 Jan 3 1970 metadata -> /dev/block/sde5
lrwxrwxrwx 1 root root 15 Jan 3 1970 mfg -> /dev/block/sdf2
lrwxrwxrwx 1 root root 15 Jan 3 1970 misc -> /dev/block/sdd1
lrwxrwxrwx 1 root root 16 Jan 3 1970 modem_a -> /dev/block/sda25
lrwxrwxrwx 1 root root 16 Jan 3 1970 modem_b -> /dev/block/sda26
lrwxrwxrwx 1 root root 15 Jan 3 1970 modemst1 -> /dev/block/sdd9
lrwxrwxrwx 1 root root 16 Jan 3 1970 modemst2 -> /dev/block/sdd10
lrwxrwxrwx 1 root root 16 Jan 3 1970 msadp_a -> /dev/block/sda27
lrwxrwxrwx 1 root root 16 Jan 3 1970 msadp_b -> /dev/block/sda28
lrwxrwxrwx 1 root root 15 Jan 3 1970 persist -> /dev/block/sdd3
lrwxrwxrwx 1 root root 15 Jan 3 1970 pg1fs -> /dev/block/sde3
lrwxrwxrwx 1 root root 15 Jan 3 1970 pg2fs -> /dev/block/sde4
lrwxrwxrwx 1 root root 15 Jan 3 1970 pmic_a -> /dev/block/sda9
lrwxrwxrwx 1 root root 16 Jan 3 1970 pmic_b -> /dev/block/sda10
lrwxrwxrwx 1 root root 15 Jan 3 1970 ramdump -> /dev/block/sde2
lrwxrwxrwx 1 root root 16 Jan 3 1970 reserve0 -> /dev/block/sda36
lrwxrwxrwx 1 root root 16 Jan 3 1970 reserve3 -> /dev/block/sdd13
lrwxrwxrwx 1 root root 15 Jan 3 1970 reserve4 -> /dev/block/sde6
lrwxrwxrwx 1 root root 15 Jan 3 1970 reserve5 -> /dev/block/sdf5
lrwxrwxrwx 1 root root 15 Jan 3 1970 rpm_a -> /dev/block/sda7
lrwxrwxrwx 1 root root 15 Jan 3 1970 rpm_b -> /dev/block/sda8
lrwxrwxrwx 1 root root 15 Jan 3 1970 sec -> /dev/block/sdd4
lrwxrwxrwx 1 root root 15 Jan 3 1970 ssd -> /dev/block/sdd2
lrwxrwxrwx 1 root root 16 Jan 3 1970 system_a -> /dev/block/sda33
lrwxrwxrwx 1 root root 16 Jan 3 1970 system_b -> /dev/block/sda34
lrwxrwxrwx 1 root root 15 Jan 3 1970 tz_a -> /dev/block/sda5
lrwxrwxrwx 1 root root 15 Jan 3 1970 tz_b -> /dev/block/sda6
lrwxrwxrwx 1 root root 16 Jan 3 1970 userdata -> /dev/block/sda35
lrwxrwxrwx 1 root root 16 Jan 3 1970 vendor_a -> /dev/block/sda31
lrwxrwxrwx 1 root root 16 Jan 3 1970 vendor_b -> /dev/block/sda32
lrwxrwxrwx 1 root root 15 Jan 3 1970 xbl_a -> /dev/block/sdb1
lrwxrwxrwx 1 root root 15 Jan 3 1970 xbl_b -> /dev/block/sdc1
```