Build and use ota_from_target_files as a module.

Bug: 63866463
Test: TreeHugger
Test: m -j otapackage
Test: `m -j otatools-package`. Check bin/ota_from_target_files is
      available. Build an OTA package with standalone otatools.
Change-Id: I533dee1c6997d233601e7f4eae637ff5743ed637