kernel: mtdsplit_minor: accept bootimage filename

kernel: mtdsplit_minor: accept bootimage filename

RouterBOOT v7 on NOR devices no longer accepts the YAFFS kernel ELF
method of booting. It will accept an NPK image named bootimage.
Adjust mtdsplit_minor to accept this second possible boot file name.
Use the conservative value of 127 for YAFFS max name length (used when
YAFFS compiled with unicode support) vs 255.

Signed-off-by: John Thomson <[email protected]>
Acked-by: Thibaut VARENE <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/16780
Signed-off-by: Robert Marko <[email protected]>