Allow custom releasetools ota

Inspired by Codeworkx

i.e.
TARGET_PROVIDES_RELEASETOOLS := true
TARGET_RELEASETOOL_OTA_FROM_TARGET_SCRIPT := vendor/commotio/tools/ota_from_target_files

Default is normal build/tools/../ota_from_target_files

Change-Id: I5745f01d5d3dbfbb4cd84e438050ca3d67edda2e
Signed-off-by: DHO <DHO@dho.im>
Merge "Allow custom releasetools ota" into jb42