busybox: init: check for zero-size *.md5 files

busybox's md5sum applet does not fail if sizeof .md5 file is 0, so
in some rare cases (impropper shutdown before update / broken RPi sdcard)
we may updateto 0byte KERNEL/SYSTEM because md5 check didn't fail

closes #2788

Conflicts:
	packages/initramfs/sysutils/busybox-initramfs/scripts/init