dfu: introduce progressive erasing

Patch adds option for progressive erase of firmware image.
When using this, flash is erased as necessary when receiving
new firmware, instead of erasing the whole image slot at once.
This is useful on some hardware (like nRF52840) that has
long erase times, to prevent long wait times at the beginning
of the DFU process.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>