kernel: mtdsplit: Add support for Mikrotik NOR firmware

kernel: mtdsplit: Add support for Mikrotik NOR firmware

The RouterBOOT bootloader does not care where the kernel lives in the SPI
flash, all that matters is that the kernel is wrapped in the custom yaffs
container as generated by kernel2minor.

This container has a fixed signature as follows:
00000000  00 00 00 01 00 00 00 01  ff ff 6b 65 72 6e 65 6c  |..........kernel|

This patch adds mtdsplit support for identifying that signature and
triggering the search for the rootfs. rootfs is expected at EB boundary since
we use wget mtd_find_rootfs_from(). We make no use of the yaffs file size
field because it contains invalid data in the image generated by kernel2minor.

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>