build: kernel2minor: work around path length limit

build: kernel2minor: work around path length limit

When building for MikroTik devices the kernel2minor tool will sometimes
fail with:

  Can't get lstat from kernel file!: No such file or directory.

This is because kernel2minor expects paths no longer than 250 chars.
To work around this the include/image-commands.mk has been modified
to copy the kernel to a temporary file (/tmp/tmp.XXXXXXXXXX) before
calling kernel2minor.

Signed-off-by: François Chavant <francois@chavant.info>
(cherry picked from commit 5a9608102b3c444d0c963255f9d016a1d5469d5d)