mb/google/skyrim: Check if SPD exists

Update the build script to check if SPD exists, and only if SPD exists
the APCB_SBR_D5.gen could be executed.

BUG=None
TEST=Build

Change-Id: Ib7b977a89d403242e8bb1f684269e70082125e88
Signed-off-by: Isaac Lee <isaaclee@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66978
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Jon Murphy <jpmurphy@google.com>
diff --git a/src/mainboard/google/skyrim/Makefile.inc b/src/mainboard/google/skyrim/Makefile.inc
index 3a91cff..0bea8a0 100644
--- a/src/mainboard/google/skyrim/Makefile.inc
+++ b/src/mainboard/google/skyrim/Makefile.inc
@@ -19,6 +19,8 @@
 ifneq ($(wildcard $(MAINBOARD_BLOBS_DIR)/APCB_SBR_D5.bin),)
 $(info APCB sources present.)
 
+ifneq ($(wildcard $(SPD_SOURCES)),)
+
 LIB_SPD_DEPS = $(SPD_SOURCES)
 
 APCB_SOURCES = $(obj)/APCB_SBR_D5.gen
@@ -32,5 +34,8 @@
 			     --spd_sources $(SPD_SOURCES) \
 			     --mem_type 'lp5'
 else
+$(info SPD sources not found.  Skipping APCB.)
+endif
+else
 $(info APCB sources not found.  Skipping APCB.)
 endif