MIPS: zboot: Add "uzImage.bin" target

uzImage.bin is vmlinuz.bin wrapped in a legacy U-Boot image. Since
the extraction code is inside the image, it does not depend on the
boot loader to extract the kernel.

Signed-off-by: Maarten ter Huurne <maarten@treewalker.org>
Cc: Alban Bedel <albeu@free.fr>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14473/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>