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/mt76/Makefile

@@ -74,8 +74,7 @@ endif
 
 define Build/Compile
 	+$(MAKE) $(PKG_JOBS) -C "$(LINUX_DIR)" \
-		ARCH="$(LINUX_KARCH)" \
-		CROSS_COMPILE="$(TARGET_CROSS)" \
+		$(KERNEL_MAKE_FLAGS) \
 		SUBDIRS="$(PKG_BUILD_DIR)" \
 		NOSTDINC_FLAGS="$(NOSTDINC_FLAGS)" \
 		modules