boot: remove the set-on-boot check (3/3)

The checkbox inside the settings is no longer needed, since
we can set each element independently.
Clean up of the patch-series.

Signed-off-by: Blechd0se <alex.christ@hotmail.de>
memory: don't try trimming if lib is missing

If the device has no access to the busybox library it could
*try* to trim forever. Fix that by checking early if the file
exists.

Signed-off-by: Blechd0se <alex.christ@hotmail.de>