Sorting supported mainboard in Kconfig.

Change-Id: If4550a1292ddd2645a730cca17d5779a4afa2088
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: http://review.coreboot.org/6134
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <gaumless@gmail.com>