board/common: skip if not SIGN_ENABLED

All Infix builds should be signed, but for some test equipment, and
during board bringup, this may be too much of a hassle.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

board/common/common.mk

@@ -1,3 +1,4 @@
+ifeq ($(SIGN_ENABLED),y)
 include $(BR2_EXTERNAL_INFIX_PATH)/board/common/uboot/uboot.mk
 
 define RAUC_POST_BUILD_INSTALL_CERT
@@ -7,3 +8,4 @@ define RAUC_POST_BUILD_INSTALL_CERT
 		cp $(crt) $(TARGET_DIR)/etc/rauc/keys/$(shell openssl x509 -hash -noout <$(crt)).0;)
 endef
 RAUC_POST_BUILD_HOOKS += RAUC_POST_BUILD_INSTALL_CERT
+endif