kernel: Make use of KERNEL_MAKE

Make use of KERNEL_MAKE in kernel packages were easily possible.
This moves some more code to common places and reduces the number of
lines.

It is defined like this:
KERNEL_MAKE = $(MAKE) $(KERNEL_MAKEOPTS)
KERNEL_MAKEOPTS = -C $(LINUX_DIR) $(KERNEL_MAKE_FLAGS)

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/kernel/mt76/Makefile

@@ -379,8 +379,7 @@ ifdef CONFIG_PACKAGE_kmod-mt7921e
 endif
 
 define Build/Compile
-	+$(MAKE) $(PKG_JOBS) -C "$(LINUX_DIR)" \
-		$(KERNEL_MAKE_FLAGS) \
+	+$(KERNEL_MAKE) $(PKG_JOBS) \
 		$(PKG_MAKE_FLAGS) \
 		M="$(PKG_BUILD_DIR)" \
 		NOSTDINC_FLAGS="$(NOSTDINC_FLAGS)" \