[DRIVERS] Headers corrected.

The doxygen comments in the headers were not correct.
[DRIVERS] A function to get a MTD device from an id has been added.

In order to operate on a specific MTD device, a function has been added to
retrieve MTD devices with a number corresponding to the registering order of
the MTD device.
[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.