build: use KERNEL_MAKE_FLAGS for kernel file compilations

The build system already defines KERNEL_CROSS which defaults to TARGET_CROSS.
Make use of this variable for kernel makefiles.

Signed-off-by: Karl Vogel <karl.vogel@gmail.com>

package/kernel/broadcom-wl/Makefile

@@ -100,8 +100,7 @@ define Package/nas/description
 endef
 
 MAKE_KMOD := $(MAKE) -C "$(LINUX_DIR)" \
-		CROSS_COMPILE="$(TARGET_CROSS)" \
-		ARCH="$(LINUX_KARCH)" \
+		$(KERNEL_MAKE_FLAGS) \
 		PATH="$(TARGET_PATH)" \
 		SUBDIRS="$(PKG_BUILD_DIR)/kmod" \