Add lunch support for TARGET_RELEASE

This replaces support for the unused
TARGET_PLATFORM_VERSION variable.

Now, if you pass three - separated
items the first is product, the
second is release and the third
is variant. If you only pass two
they're still product-variant
and the build system will choose
a reasonable default for release.

Test: run lunch with two and three items, confirmed values in the build banner
Change-Id: I128177d96ffe81b79b6945a24ebf37861c3b25fc