batman-adv: use KERNEL_MAKE_FLAGS for kernel module compilation

It is easier to use the global define than to manually keep track of the
changes in the various kernel module makefiles in the main OpenWrt
repository and feeds.

Signed-off-by: Sven Eckelmann <sven@narfation.org>

batman-adv/Makefile

@@ -4,7 +4,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=batman-adv
 PKG_VERSION:=2020.1
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=https://downloads.open-mesh.org/batman/releases/batman-adv-$(PKG_VERSION)
@@ -75,8 +75,7 @@ NOSTDINC_FLAGS = \
 
 define Build/Compile
 	$(MAKE) $(PKG_JOBS) -C "$(LINUX_DIR)" \
-		ARCH="$(LINUX_KARCH)" \
-		CROSS_COMPILE="$(TARGET_CROSS)" \
+		$(KERNEL_MAKE_FLAGS) \
 		M="$(PKG_BUILD_DIR)/net/batman-adv" \
 		$(PKG_EXTRA_KCONFIG) \
 		EXTRA_CFLAGS="$(PKG_EXTRA_CFLAGS)" \