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/include/cbfs_core.h

@@ -82,7 +82,7 @@
     so we can easily find it. */
 
 #define CBFS_HEADER_MAGIC  0x4F524243
-#if CONFIG_TARGET_I386
+#if CONFIG_ARCH_X86
 #define CBFS_HEADPTR_ADDR 0xFFFFFFFC
 #endif
 #define VERSION1 0x31313131