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/bin/lpgcc

@@ -102,7 +102,7 @@ while [ $# -gt 0 ]; do
 	shift
 done
 
-if [ "$CONFIG_TARGET_I386" = "y" ]; then
+if [ "$CONFIG_ARCH_X86" = "y" ]; then
   _ARCHINCDIR=$_INCDIR/i386
   _ARCHLIBDIR=$_LIBDIR/i386
 fi