UART no response after upgrade to Buster (similar issue to #291)

This concerns a system that's been in production since 2016. My client recently asked for some new features. To do that, I needed to upgrade from Debian Jessie to Buster and from Python 2 to 3.

## The problem
The system uses `/dev/ttyO0` and `/dev/ttyO1` for ModBus serial control at 19200 baud. Since the upgrade, these ports can be opened and written to but they return no response.

## Jessie serial configuration
Under Jessie, the ports were initialized in `/etc/rc.local` with:

```bash
echo BB-UART1 > /sys/devices/bone_capemgr.9/slots
echo BB-UART2 > /sys/devices/bone_capemgr.9/slots
```

and /dev/ttyO0 was released from use as a login console by commenting out the following lines in `/etc/init/serial.conf`
```
#respawn
#exec /sbin/getty 115200 ttyO0
```

## Buster serial configuration
Under Buster, the overlay system is changed and `/etc/rc.local` and `/etc/init/serial.conf` are obsoleted in favor of `systemd`, so I'm enabling UART's with:
```bash
sudo config-pin P9.24 uart # UART1_TXD
sudo config-pin P9.26 uart # UART1_RXD
sudo config-pin P9.21 uart # UART2_TXD
sudo config-pin P9.22 uart # UART2_RXD
```
and disabling the login console mapping with:
```
sudo systemctl mask serial-getty@ttyO0.service
```

## Buster system report from /opt/scripts/tools/version.sh
```
signtex@beaglebone:~$ sudo /opt/scripts/tools/version.sh
git:/opt/scripts/:[]
eeprom:[A335BNLT00C03416BBBK2413]
model:[TI_AM335x_BeagleBone_Black]
dogtag:[BeagleBoard.org Debian Buster IoT Image 2022-12-01]
bootloader:[microSD-(push-button)]:[/dev/mmcblk0]:[U-Boot SPL 2019.04-g923f8b8 (Jan 02 2022 - 19:05:15 +0000)]:[location: dd MBR]
bootloader:[microSD-(push-button)]:[/dev/mmcblk0]:[U-Boot 2019.04-g923f8b8]:[location: dd MBR]
UBOOT: Booted Device-Tree:[am335x-boneblack-uboot-univ.dts]
UBOOT: Loaded Overlay:[AM335X-PRU-RPROC-4-19-TI-00A0.kernel]
UBOOT: Loaded Overlay:[BB-ADC-00A0.kernel]
UBOOT: Loaded Overlay:[BB-BONE-eMMC1-01-00A0.kernel]
UBOOT: Loaded Overlay:[BB-HDMI-TDA998x-00A0.kernel]
kernel:[4.19.94-ti-r73]
nodejs:[v10.24.0]
/boot/uEnv.txt Settings:
uboot_overlay_options:[enable_uboot_overlays=1]
uboot_overlay_options:[uboot_overlay_pru=AM335X-PRU-RPROC-4-19-TI-00A0.dtbo]
uboot_overlay_options:[enable_uboot_cape_universal=1]
pkg check: to individually upgrade run: [sudo apt install --only-upgrade ]
pkg:[bb-cape-overlays]:[4.14.20210821.0-0~buster+20210821]
pkg:[bb-customizations]:[1.20221108.0-0~buster+20221108]
pkg:[bb-usb-gadgets]:[1.20220816.0-0~buster+20220816]
pkg:[bb-wl18xx-firmware]:[1.20221130.4-0~buster+20221130]
pkg:[kmod]:[26-1]
pkg:[librobotcontrol]:[1.0.5-git20200715.0-0~buster+20200716]
pkg:[firmware-ti-connectivity]:[20190717-2rcnee1~buster+20200305]
groups:[debian : debian adm kmem dialout cdrom floppy audio dip video plugdev users systemd-journal input bluetooth netdev i2c gpio admin tisdk weston-launch cloud9ide]
cmdline:[console=ttyS0,115200n8 bone_capemgr.uboot_capemgr_enabled=1 root=/dev/mmcblk0p1 ro rootfstype=ext4 rootwait coherent_pool=1M net.ifnames=0 lpj=1990656 rng_core.default_quality=100 quiet]
dmesg | grep remote
[ 54.142160] remoteproc remoteproc0: wkup_m3 is available
[ 54.210850] remoteproc remoteproc0: powering up wkup_m3
[ 54.210884] remoteproc remoteproc0: Booting fw image am335x-pm-firmware.elf, size 217148
[ 54.211173] remoteproc remoteproc0: remote processor wkup_m3 is now up
[ 57.564726] remoteproc remoteproc1: 4a334000.pru is available
[ 57.567096] remoteproc remoteproc2: 4a338000.pru is available
dmesg | grep pru
[ 57.564726] remoteproc remoteproc1: 4a334000.pru is available
[ 57.564922] pru-rproc 4a334000.pru: PRU rproc node pru@4a334000 probed successfully
[ 57.567096] remoteproc remoteproc2: 4a338000.pru is available
[ 57.567240] pru-rproc 4a338000.pru: PRU rproc node pru@4a338000 probed successfully
dmesg | grep pinctrl-single
[ 1.066060] pinctrl-single 44e10800.pinmux: 142 pins, size 568
dmesg | grep gpio-of-helper
[ 1.079463] gpio-of-helper ocp:cape-universal: ready
lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
END
```

No repository file or test is named. Start by reviewing the Buster `config-pin` commands, `serial-getty@ttyO0.service` masking, `/dev/ttyO0` and `/dev/ttyO1`, and the diagnostics from `version.sh`; compare them with the Jessie configuration. Done means identifying a reproducible cause and confirming that both ModBus UART ports receive responses after the upgrade.

Written by the indexing model from the issue text.