ar71xx: Enable mtdsplit support for RB SPI NOR devices

This patch enables mtdsplit for the MikroTik subtarget.
It converts mach-rbspi.c to use a single "firmware" partition.
Finally, it converts the SPI NOR profile to metadata images:
the original combined-image format is gone, the images are now
"standard" sysupgrade images with metadata appended.

Note: kernel2minor apparently pads the kernel container to erase-block
boundary, but this is undocumented behaviour, so we do not rely on it
and call pad-to anyway.

Note: in platform.sh, the boards are tested last in platform_check_image()
as this should eventually become the "*)" default case when more devices
switch to metadata images.

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