x86: remove cpu_incs as romstage Make variable

When building up which files to include in romstage there
were both 'cpu_incs' and 'cpu_incs-y' which were used to
generate crt0.S. Remove the former to settle on cpu_incs-y
as the way to be included.

BUG=chrome-os-partner:44827
BRANCH=None
TEST=Built rambi. No include file changes.

Change-Id: I8dc0631f8253c21c670f2f02928225ed5b869ce6
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/11494
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/src/mainboard/emulation/qemu-i440fx/Makefile.inc b/src/mainboard/emulation/qemu-i440fx/Makefile.inc
index 7423b2b..f9cf252 100644
--- a/src/mainboard/emulation/qemu-i440fx/Makefile.inc
+++ b/src/mainboard/emulation/qemu-i440fx/Makefile.inc
@@ -1,3 +1,3 @@
-cpu_incs += $(src)/mainboard/emulation/qemu-i440fx/cache_as_ram.inc
+cpu_incs-y += $(src)/mainboard/emulation/qemu-i440fx/cache_as_ram.inc
 ramstage-y += northbridge.c
 ramstage-y += fw_cfg.c