fastboot: Clean up the image array a bit.

This makes the following changes to the images array:
 * The "optional_if_no_partition" flag is removed, as it was a temporary
   workaround for super.img.
 * The "flashall" flag is removed.
 * The "needed_for_fastbootd" flag is now an ImageType enum.

Bug: N/A
Test: fastboot builds, flashall works
Change-Id: Id000ea20cf83639cec2c9ae94f28c799b34606f1