base-files: sysupgrade cleanup

base-files: sysupgrade cleanup

Some functions only used by stage2 are moved there from common.sh.

One piece that could still use more cleanup is platform_pre_upgrade: many
targets reference files from there are aren't available in the ramfs, so
we need to evaluate it before the switch; conversely, flash writes happen
in that function on some targets. Targets that do the latter should be
fixed eventually to use platform_do_upgrade for that purpose.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>