rockchip: cmd: add rkmtd command

The command rkmtd creates a virtual block device to transfer
Rockchip boot block data to and from NAND with block orientated
tools like "ums" and "rockusb".

It uses the Rockchip MTD driver to scan for boot blocks and copies
data from the first block in a GPT formated virtual disk.
Data must be written in U-boot "idbloader.img" format and start at
partition "loader1" offset 64. The data header is parsed
for length and offset. When the last sector is received
it erases up to 5 erase blocks on NAND and writes bootblocks
in a pattern depending on the NAND ID. Data is then verified.
When a block turns out bad the block header is discarded.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>