build: Add use of custom releasetools

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>
Allow override of and disabling device asserts

* including multi-device support.

Set in board file with TARGET_OTA_ASSERT_DEVICE and TARGET_OTA_ASSERT_SKIP, respectively..

Change-Id: I27f89f440bdc4982dcac35631496c865f2657c83

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>