Adding concept of release versions for targets.

Previously, the condition for including a target in a release was decided
by a 'release' key being set to 'true' in hal/targets.json. This doesn't
have enough granularity when we release multiple versions of mbed. This PR
changes the 'release' key to an array of strings, where each member is a
version that the target supports. Currently the valid versions are '2' and
'5'.

This PR also adds more robust checking for invalid target configurations
in a release. This is enforced whenever the release list is built from the
data, preventing invalid targets from coming into the release.

Finally, it updates the build_release.py script to use the new api for
fetching release targets.

Author: bridadan