releasetool: add OSIP partitions specifics

- Declare OSIP as a new partition type
- Standard boot.img update is not possible for OSIP partitions
We are simply removing it on AOSP handling, and let the board handle
it in its own releasetool.py

Change-Id: Ie19fc3f6a0ca42e2eda264904b20fceeddbeaf53