[COMMANDS] Basic flash commands have been added.

Basic flash commands have been added which are:
- help, to get the help with the flash commands.
- list, to get the list of registered MTD devices.
- read, to read a MTD device. The offset and the number of bytes to read can be
specified on the command line.
- erase, to erase blocks from a MTD device. The offset and the length to erase
can be specified on the command line. The length must be in bytes and block-
size aligned.
- write, to write on a MTD device. The offset and the bytes to write must be
specified on the command line.

Signed-off-by: Jimmy Durand Wesolowski <jimmy.durand-wesolowski@openwide.fr>