[COMMANDS] Add a blockdev dump8 command.

This patch adds a dump8 command for block devices, allowing to
read and pring data from it.
Arguments:
- the device name (mandatory)
- the amount of data in bytes to read. If not specified, it will
  take the device default block size.
- the offset in bytes.

Signed-off-by: Jimmy Durand Wesolowski <jimmy.durand-wesolowski@openwide.fr>
Signed-off-by: Anup Patel <anup@brainfault.org>