toolchain: kernel-headers: fix check target for external Git trees

Executing following command currently fails:

 $ make toolchain/kernel-headers/{download,check} V=sc FIXUP=1
 ...
 include/kernel-version.mk:11: *** Missing kernel version/hash file for . Please create include/kernel-.  Stop.

So lets fix it by adding the necessary missing KERNEL_PATCHVER variable.

That additional kernel-build.mk include is needed to add another set of
missing variables:

 $ make toolchain/kernel-headers/{download,check} V=sc FIXUP=1
 ...
 Makefile:115: *** ERROR: Unknown pack format for file tmp/dl/.  Stop.

Fixes: 0765466a42f4 ("kernel: split kernel version to dedicated files")
Signed-off-by: Petr Štetiar <[email protected]>

toolchain/kernel-headers/Makefile

@@ -23,7 +23,10 @@ ifneq ($(call qstrip,$(CONFIG_KERNEL_GIT_CLONE_URI)),)
   PKG_SOURCE_VERSION:=$(call qstrip,$(CONFIG_KERNEL_GIT_REF))
   PKG_MIRROR_HASH:=$(call qstrip,$(CONFIG_KERNEL_GIT_MIRROR_HASH))
 ifdef CHECK
+  PLATFORM_DIR:=$(firstword $(wildcard $(TOPDIR)/target/linux/feeds/$(BOARD) $(TOPDIR)/target/linux/$(BOARD)))
+  include $(PLATFORM_DIR)/Makefile
   include $(INCLUDE_DIR)/kernel-version.mk
+  include $(INCLUDE_DIR)/kernel-build.mk
   PKG_VERSION:=$(LINUX_VERSION)
 else
   PKG_SOURCE:=$(LINUX_SOURCE)