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 branch 'platforms' of https://github.com/ladislav/r3 into community