Fix #499: Don't generate weston.ini from the deviceinfo anymore (#739)

@drebrez deserves much credit for this one for all the testing,
bisecting and for fixing everything. Thank you very much!
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* devices which need a custom weston.ini ship it with a install_if
  subpackage, so it only gets installed when weston is installed. This
  sounds complicated, but is actually pretty clean in the APKBUILD.
* postmarketos-ui-weston: has a weston.ini.default, which enables
  xwayland and uses fbdev as backend (because that's what most
  devices use!). It defaults to the weston.ini.default if there is no
  weston.ini (as installed by the device package).
* changed spaces to tabs for consistency, general minor refactoring of
  device-APKBUILDs