procd: firstboot nand/ubi takeover support

it is now possible to flash a initramfs kernel and a sysupgrade tar file inside the ubi partition. on first boot, the takeover script will find the tar file, extract and finally sysupgrade it. this allows us to flash owrt/ubi images in a 2 phase setup using ODM webuis that are not ubi aware. this is needed by some mediatek and brokencom devices.

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43505 3c298f89-4303-0410-b956-a3cf2f4a3e73