drivers: flash: shell: add "flash copy" command

Add a flash copy command, capable of copying a region in one
flash device to a region on the same or another flash device. The
destination is erased prior to copying.

This is useful for evaluating mcuboot on devices with little
on-chip resources, or devices that are incapable of running more
elaborate image management services (e.g. via bluetooth or
networking).

Additionally, it's useful for evaluating mcuboot on devices with
one or more images stored on external spi flash.

The command syntax is
flash copy <src_dev> <dst_dev> <src_offs> <dest_offs> <size>

E.g.
flash copy flash@0 flash-controller@abcd1234 0x1234 0x5678 21012
Copied 21012 bytes from flash@0:1234 to \
  flash-controller@abcd1234:5678

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>