Move ARCH_* from board/Kconfig to cpu or soc Kconfig.

CONFIG_ARCH is a property of the cpu or soc rather than a property of the
board. Hence, move ARCH_* from every single board to respective cpu or soc
Kconfigs. Also update abuild to ignore ARCH_ from mainboards.

Change-Id: I6ec1206de5a20601c32d001a384a47f46e6ce479
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: http://review.coreboot.org/5570
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/mainboard/iwave/iWRainbowG6/Kconfig

@@ -3,7 +3,6 @@ if BOARD_IWAVE_RAINBOW_G6
 # TODO: move options to chipset components as appropriate
 config BOARD_SPECIFIC_OPTIONS # dummy
 	def_bool y
-	select ARCH_X86
 	select CPU_INTEL_SOCKET_441
 	select NORTHBRIDGE_INTEL_SCH
 	select SOUTHBRIDGE_INTEL_SCH