Let device maintainer define BLISS_VERSION_MAINTENANCE

Not all devices are created equal. Let's let maintainers add their device maintenance type themselves.
Examples:
$ export BLISS_VERSION_MAINTENANCE=Stable

or from device config:

BLISS_VERSION_MAINTENANCE := Stable
Change-Id: I1931ec6097a258b2dd8951d89d21cc54545d7bf1
1 file changed