add --block flag to ota_from_target_files

Add the --block flag to this script to control whether block-based OTA
packages are generated (defaults to off).  Make the full OTA package
produced by "make otapackage" continue to produce a block-based OTA.

Also fix a problem where block incremental OTAs didn't ever succeed,
and the --no_signing option never worked.

Change-Id: I610d0b4abed4b8b65fbe8ce0abaeec6cf52e14a1