Move compatible strings from board structs into separate table. This
allows for several board compatibles to match to the same board in case
e.g. only the flash size / partitions differ.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43341 3c298f89-4303-0410-b956-a3cf2f4a3e73