pmb.flasher: Improve flash_lk2nd action

* Check that we're not already running lk2nd as flashing boot partition
  inside lk2nd is different to flashing boot partition outside. We could
  improve this in the future to use "flash lk2nd lk2nd.img" as
  documented in the file.
* Grab the lk2nd package from the device package and install that. The
  device package is expected to have a dependency on the correct lk2nd
  package.
* Remove some log message in unusual styles for pmbootstrap.
* Group flash_lk2nd action together with the other flash actions and use
  string comparison.

See also: https://gitlab.com/postmarketOS/pmaports/-/issues/2074



Reviewed-by: Oliver Smith <ollieparanoid@postmarketos.org>
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20230605104026.744005-1-luca@z3ntu.xyz%3E