[Task]: Improve `armbian-install` when using multiple partitions

## Task description

Forum topic for reference: https://forum.armbian.com/topic/41699-how-to-debugfix-armbian-config-for-kernel-update/

Quoting the issues @hi-ko had from https://github.com/armbian/build/pull/6828#issuecomment-2207408363

> ## short version
>
> having something like `parted ${ARMBIAN_PARTITION} set 1 boot on` in arbian-install would have avoided my issue discussed in the forum.
> ## long version
>
> I had three issues, described in the forum thread:
>
> - inconsistent /etc/armbian-release file due to renaming BOARDFAMILY to rockchip64 which broke armbian-config / armbian-install
> - unbootable system after running armbian-config to switch/update the kernel (which then may not fit to the bootloader config)
> - unbootable system after running armbian-install from sdcard to install into a manual wiped partition due to missing boot flag on that partition
>
> supporting multiple partitions should be always seen as best practice since it may be required to replace the armbian partition from time to time (since there is no such thing like a do-release-upgrade for now). Having one or more data partitions, which will guarantee to survive, could be a life saver then.
>
> That does not necessarily mean, that all scenarios should be supported by the armbian-install. Experienced people have no problems in rsyning the sdcard to a custom partition or to wipe an already used partition but they may need to know the steps expected to be done and the knowledge how to configure/check the bootloader. at least for me u-boot is new territory. I will need to dig into u-boot and your scripts.
>
> So my suggestion to start with is:
>
> we should document (I could help on that)
>
> - the steps to be done manually if _not_ using armbian-install
> - the _not_ supported scenarios by armbian-install
>
>
> features I would like to have are:
>
> - run armbian-install with parameters / without dialogue (e.g. `armbian-install --bootloader-mtd-flash /dev/nvme0n1p2 /mnt/armbian/boot/`)

## Task list

- [ ] Improve `armbian-install` when using multiple partitions by (maybe?) checking/adding a boot flag to the respective partition
- [ ] Add documentation as quoted above to https://docs.armbian.com/User-Guide_Getting-Started/#how-to-install-to-emmc-sata-nvme-usb (GitHub: https://github.com/armbian/documentation/blob/master/docs/User-Guide_Getting-Started.md )
- [ ] the steps to be done manually if _not_ using armbian-install
- [ ] the _not_ supported scenarios by armbian-install
- [ ] (maybe) Add possibility to run armbian-install with parameters / without dialogue (e.g. `armbian-install --bootloader-mtd-flash /dev/nvme0n1p2 /mnt/armbian/boot/`)