Change TARGET_I386 to ARCH_X86

This renames TARGET_I386 to ARCH_X86 to make it more uniform with
other parts of the codebase, e.g. cbfs_core.h from cbfstool.

Change-Id: I1babcc941245ed1dde0478a21828766759373a42
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: http://review.coreboot.org/1961
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Dave Frodin <dave.frodin@se-eng.com>

payloads/libpayload/arch/i386/Config.in

@@ -20,7 +20,7 @@
 ## MA 02111-1307 USA
 ##
 
-if TARGET_I386
+if ARCH_X86
 
 config ARCH_SPECIFIC_OPTIONS # dummy
 	def_bool y