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/gpio-button-hotplug/Makefile

@@ -32,14 +32,8 @@ define KernelPackage/gpio-button-hotplug/description
  an overkill for OpenWrt simple needs.
 endef
 
-MAKE_OPTS:= \
-	$(KERNEL_MAKE_FLAGS) \
-	M="$(PKG_BUILD_DIR)"
-
 define Build/Compile
-	$(MAKE) -C "$(LINUX_DIR)" \
-		$(MAKE_OPTS) \
-		modules
+	$(KERNEL_MAKE) M="$(PKG_BUILD_DIR)" modules
 endef
 
 $(eval $(call KernelPackage,gpio-button-hotplug))