Make build platform handling more robust

Change platforms.r to use pairs of "minor" platform number and value.

Add the Android platform as well as old R2 platforms.

Adapt make-boot.r accordingly.

Add a sanity check for the existence of a platform.

Fix handling of the "system/product" name.

This contribution was originally written by Richard Smolak.
Merge pull request #93 from ladislav/platforms

Enhanced build platform specification - easier to maintain. Add various other platforms, including Android.