tests/Makefile.inc: Build utilities in separate directory

Utilities like kconfig/conf now will be built inside tests build tree.
It will eliminate possible colisions of target names when using test
framework in more than one place (see CB:70110)

Signed-off-by: Jakub Czapiga <jacz@semihalf.com>
Change-Id: I4c1eb901c921f4ec6ee8985b154362153c5fd0e7
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70359
Reviewed-by: Jan Dabros <jsd@semihalf.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

tests/Makefile.inc

@@ -8,6 +8,8 @@ else
 testobj := $(obj)/tests
 endif
 
+objutil := $(testobj)/util
+
 include $(top)/tests/Makefile.common
 
 # Enable GDB debug build if requested