cmd: mtdparts: support runtime generated mtdparts

Some CPUs contains boot ROM code capable reading first few blocks
(where SPL resides) of NAND flash and executing it. It is wise to
create separate partition here for SPL. As block size depends on
NAND chip used, we could either use worst case (biggest) partition
size or base its size on actual block size. This patch adds support
for the latter option.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>