fastboot: Resize logical partitions when using the flash command.

If an image size changes slightly in between "flashall" and some change
in the build, it's inconvenient to have to reflash the whole device again.
This patch resizes partitions when using a normal flash command, to
improve the developer workflow.

Bug: N/A
Test: fastboot flash system
Change-Id: I349364a4742a0c15748f545fcfda946107720d45