Move the non-GUI component of update_checksums() to
/etc/functions so it can be reused outside of gui-init.
Add check that /boot/kexec_default_hashes.txt exists before parsing
it, since doesn't exist if there's no default boot target set yet.
Eliminates spurious error text and/or premature exit depending on
state of errexit.
Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>