flashcp: Add option -A/--erase-all

Sometimes there's a need to erase the whole device when
programming an image, and not just the area under the
image itself.

Therefore, option -A/--erase-all has been added which
erases the whole device before writing the image.

Signed-off-by: Andrij Abyzov <drolevar@gmail.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>