tests/Makefile.common: Allow to override tests defconfig

Some tools based on test framework might require defconfig other
than one set in tests framework.

Signed-off-by: Jakub Czapiga <jacz@semihalf.com>
Change-Id: If53b9a54ef7389dd979dfe772e6946439f7d6a62
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70109
Reviewed-by: Jan Dabros <jsd@semihalf.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/tests/Makefile.common b/tests/Makefile.common
index 8277d5f..8cb39a0 100644
--- a/tests/Makefile.common
+++ b/tests/Makefile.common
@@ -18,7 +18,7 @@
 OBJCOPY ?= objcopy
 OBJDUMP ?= objdump
 
-TEST_DEFAULT_CONFIG := $(top)/configs/config.emulation_qemu_x86_i440fx
+TEST_DEFAULT_CONFIG ?= $(top)/configs/config.emulation_qemu_x86_i440fx
 TEST_DOTCONFIG := $(testobj)/.config
 TEST_KCONFIG_AUTOHEADER := $(testobj)/config.src.h
 TEST_KCONFIG_AUTOCONFIG := $(testobj)/auto.conf