commands: md, mw: add the '-x' option (swap bytes)

[Antony Pavlov]

Most of my MIPS boards run barebox in big-endian mode, but
sometimes I need see state of little-endian registers
(e.g. registers of PCI-connected peripherals) and it is not
very handy to swap bytes manually.

This patch series adds to memory commands md and mw the ability
to swap bytes "on the fly".

[PATCH 1/3] mem: add the swab (swap bytes) option to
[PATCH 2/3] commands: md: add the '-x' option (swap bytes)
[PATCH 3/3] commands: mw: add the '-x' option (swap bytes)