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

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 <ynezz@true.cz>
(cherry picked from commit 6b7f1ffbad732c5e1818f9387c02dda109c2748b)