fastboot: Implement flashing session counter

The fastboot flash command that writes an image to a partition works in
several steps:

1 - Retrieve the maximum size the device can download through the
    "max-download-size" variable

2 - Retrieve the partition type through the "partition-type:%s" variable,
    that indicates whether or not the partition needs to be erased (even
    though the fastboot client has minimal support for that)

3a - If the image is smaller than what the device can handle, send the image
     and flash it.

3b - If the image is larger than what the device can handle, create a
     sparse image, and split it in several chunks that would fit. Send the
     chunk, flash it, repeat until we have no more data to send.

However, in the 3b case, the subsequent transfers have no particular
identifiers, the protocol just assumes that you would resume the writes
where you left it.

While doing so works well, it also means that flashing two subsequent
images on the same partition (for example because the user made a mistake)
would not work withouth flashing another partition or rebooting the board,
which is not really intuitive.

Since we have always the same pattern, we can however maintain a counter
that will be reset every time the client will retrieve max-download-size,
and incremented after each buffer will be flashed, that will allow us to
tell whether we should simply resume the flashing where we were, or start
back at the beginning of the partition.

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