Implement sysupgrade for Raspberry Pi, similar to the way it is done on x86:
The config files are saved in the boot partition and moved to where they are
normally expected in preinit.

Also add optional gzip compression for the SD card image, since this can save
a lot of space (76M vs 6M), also similar to x86.

Signed-off-by: Bruno Randolf <br1@einfach.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46347 3c298f89-4303-0410-b956-a3cf2f4a3e73