flashcp: Add write last option.

flashcp: Add write last option.

This option is useful for power fail resilient bootloader updates, for
systems that check only the partition header for validity before
attempting to load the bootloader. For example zynq.

Postponing the write of a specified amount of bytes at the beginning of
a bootloader makes sure a written bootloader slot is not considered
valid until all the data is written.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>