Revert "tests: Allow specifying vboot source directory"

This reverts commit 7713a2f295d9ed9a7023a78e085ce190ee1203fe.

Reason for revert: breaks main branch

Change-Id: I2749bea9369c222e510b838e278c7797d5dce56e
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78852
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Patrick Georgi <patrick@coreboot.org>
diff --git a/Makefile.inc b/Makefile.inc
index 2f810d5..e382518 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -460,6 +460,7 @@
 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