tests: Allow specifying vboot source directory

Respect VBOOT_SOURCE while including generic headers.

BUG=none
TEST=make clean-unit-tests &&
     VBOOT_SOURCE=/path/to/vboot_reference/ make unit-tests -j
TEST=make clean-unit-tests && make unit-tests -j
BRANCH=none

Change-Id: Id3bb3726c91167d2dd648d748763a3948787f28d
Signed-off-by: roccochen@chromium.com <roccochen@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78849
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/Makefile b/Makefile
index 29597c8..b0a99146 100644
--- a/Makefile
+++ b/Makefile
@@ -15,6 +15,7 @@
 
 additional-dirs :=
 
+VBOOT_SOURCE ?= $(abspath 3rdparty/vboot)
 VBOOT_HOST_BUILD ?= $(abspath $(objutil)/vboot_lib)
 
 COREBOOT_EXPORTS := COREBOOT_EXPORTS
diff --git a/Makefile.inc b/Makefile.inc
index e382518..2f810d5 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -460,7 +460,6 @@
 endif
 
 CPPFLAGS_common := -Isrc -Isrc/include -Isrc/commonlib/include -Isrc/commonlib/bsd/include -I$(obj)
-VBOOT_SOURCE ?= 3rdparty/vboot
 CPPFLAGS_common += -I$(VBOOT_SOURCE)/firmware/include
 CPPFLAGS_common += -include $(src)/include/kconfig.h
 CPPFLAGS_common += -include $(src)/include/rules.h
diff --git a/tests/Makefile.common b/tests/Makefile.common
index e51d253..9cc79d7 100644
--- a/tests/Makefile.common
+++ b/tests/Makefile.common
@@ -36,7 +36,7 @@
 
 TEST_INCLUDES += -I$(src) -I$(src)/include -I$(src)/commonlib/include \
 	-I$(src)/commonlib/bsd/include -I$(src)/arch/x86/include \
-	-I$(top)/3rdparty/vboot/firmware/include
+	-I$(VBOOT_SOURCE)/firmware/include
 
 # Path for Kconfig autoheader
 TEST_INCLUDES += -I$(dir $(TEST_KCONFIG_AUTOHEADER))