kbuild: Add MIPS specific files to generated package.

kbuild: Add MIPS specific files to generated package.

A lot of 64-bit systems supported by Linux/MIPS have boot firmware or
bootloaders that only understand 32-bit ELF files, and as such, the vmlinux.32
target exists to support these systems.  Therefore, it'd be nice if the tar-pkg
target recognised this, and included the right version when packaging up a
binary of the kernel.

This updates buildtar to support MIPS targets.  MIPS may use 'vmlinux'
or 'vmlinux.32' depending on the target system.  This uses 'vmlinux.32'
in preference to 'vmlinux' where present (although I should check which
is newer), including either file as /boot/vmlinux-${version}.

Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/1673/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>