KM: add the KM_UBI_PART_BOOT_OPTS #define

This define can be used if the ubi boot partition (defined for all
Keymile boards with KM_UBI_PARTITION_NAME_BOOT #define to ubi0) needs
some additionnal boot options.

This is the case for the kmp204x boards since u-boot does not support
NAND Flash subpage accesses on this platform, an additionnal argument
that defines the VID offstet must be given to the kernel.

The UBI cmd line option now looks like this "ubi.mtd=ubi0,2048" on this
platform.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Reviewed-by: York Sun <yorksun@freescale.com>