tests/Makefile.inc: Move generated headers to corresponding build dir

Test-local config override headers were generated to paths missing
/tests/ infix, thus creating divergent tree in build output directory.
This patch fixes it moving generated config headers to the test-local
build directory.

Signed-off-by: Jakub Czapiga <jacz@semihalf.com>
Change-Id: Ic5f3ba287ba3e9f5897cbaac64e88c2809f52d73
Reviewed-on: https://review.coreboot.org/c/coreboot/+/54917
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
diff --git a/tests/Makefile.inc b/tests/Makefile.inc
index 6cf92fa..3473ef7 100644
--- a/tests/Makefile.inc
+++ b/tests/Makefile.inc
@@ -100,7 +100,7 @@
 define TEST_CC_template
 
 # Generate custom config.h redefining given symbols
-$(1)-config-file := $(obj)/$(1)/config.h
+$(1)-config-file := $(testobj)/$(1)/config.h
 $$($(1)-config-file): $(TEST_KCONFIG_AUTOHEADER)
 	mkdir -p $$(dir $$@)
 	printf '// File generated by tests/Makefile.inc\n// Do not change\n' > $$@