build: add KERNEL_MAKE and KERNEL_MAKE_FLAGS variables and move to kernel.mk

This allows packages to use kernel make options without the forced
-C $(LINUX_DIR). It also makes it more clear that it to be called from
kernel module packages directly.

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/kernel.mk

@@ -87,6 +87,35 @@ else
   LINUX_KARCH := $(ARCH)
 endif
 
+KERNEL_MAKE = $(MAKE) $(KERNEL_MAKEOPTS)
+
+KERNEL_MAKE_FLAGS := \
+	HOSTCFLAGS="$(HOST_CFLAGS) -Wall -Wmissing-prototypes -Wstrict-prototypes" \
+	CROSS_COMPILE="$(KERNEL_CROSS)" \
+	ARCH="$(LINUX_KARCH)" \
+	KBUILD_HAVE_NLS=no \
+	KBUILD_BUILD_USER="$(call qstrip,$(CONFIG_KERNEL_BUILD_USER))" \
+	KBUILD_BUILD_HOST="$(call qstrip,$(CONFIG_KERNEL_BUILD_DOMAIN))" \
+	KBUILD_BUILD_TIMESTAMP="$(KBUILD_BUILD_TIMESTAMP)" \
+	KBUILD_BUILD_VERSION="0" \
+	HOST_LOADLIBES="-L$(STAGING_DIR_HOST)/lib" \
+	CONFIG_SHELL="$(BASH)" \
+	$(if $(findstring c,$(OPENWRT_VERBOSE)),V=1,V='') \
+	$(if $(PKG_BUILD_ID),LDFLAGS_MODULE=--build-id=0x$(PKG_BUILD_ID)) \
+	KERNELRELEASE=$(LINUX_VERSION) \
+	cmd_syscalls=
+
+ifeq ($(call qstrip,$(CONFIG_EXTERNAL_KERNEL_TREE))$(call qstrip,$(CONFIG_KERNEL_GIT_CLONE_URI)),)
+  KERNEL_MAKEOPTS += \
+	KERNELRELEASE=$(LINUX_VERSION)
+endif
+
+KERNEL_MAKEOPTS := -C $(LINUX_DIR) $(KERNEL_MAKE_FLAGS)
+
+ifdef CONFIG_USE_SPARSE
+  KERNEL_MAKEOPTS += C=1 CHECK=$(STAGING_DIR_HOST)/bin/sparse
+endif
+
 define KernelPackage/Defaults
   FILES:=
   AUTOLOAD:=