sparse: Implement several chunks flashing

The fastboot client will split the sparse images into several chunks if the
image that it tries to flash is bigger than what the device can handle.

In such a case, the bootloader is supposed to retain the last offset to
which it wrote to, so that it can resume the writes at the right offset
when flashing the next chunk.

Retain the last offset we used, and use the session ID to know if we need
it or not.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>