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>

package/base-files/files/sbin/sysupgrade

@@ -1,7 +1,5 @@
 #!/bin/sh
 
-[ "$1" = "nand" ] && exec /lib/upgrade/stage2 "$2" "$3"
-
 . /lib/functions.sh
 . /lib/functions/system.sh