fastboot: Add --disable-verity and --disable-verification options.

This can be used to disable verity and/or verification when flashing a
build to a device. It works with both 'fastboot flashall' and
'fastboot flash vbmeta /path/to/vbmeta.img'.

Bug: 62903976
Test: Manually tested.
Change-Id: Iad22d42a9dd5befd70ecd0224803721a10a28d90