common.make: Use dedicated env variable

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/scripts/common.make b/scripts/common.make
index 21385a3..d102a97 100644
--- a/scripts/common.make
+++ b/scripts/common.make
@@ -45,8 +45,8 @@
 endif
 
 THIRDPARTY_DIR = $(MBEDTLS_PATH)/tf-psa-crypto/drivers
-include $(MBEDTLS_PATH)/tf-psa-crypto/drivers/everest/Makefile.inc
-include $(MBEDTLS_PATH)/tf-psa-crypto/drivers/p256-m/Makefile.inc
+include $(THIRDPARTY_DIR)/everest/Makefile.inc
+include $(THIRDPARTY_DIR)/p256-m/Makefile.inc
 LOCAL_CFLAGS+=$(THIRDPARTY_INCLUDES)
 
 ifdef PSASIM