Stress the importance of testing the platform variable

[davelab6]

From http://lists.sugarlabs.org/archive/sugar-devel/2016-May/052710.html and replies, it seems some dependencies must be changed based on XO model and on 32 or 64 bit architechture.

During GCI, Walter set up a platform variable that activities can test to determine the environment. This will become increasingly important to developers, so should be stressed (and perhaps part of Hello World.)

The XO-1.75 needs ARMv7 binaries, while older XOs used x86. Activity bundles should include .so files for each supported architecture. There is not yet a formal bundle structure for handling multi-arch, as it was designed for platform-independent code like Python, but since a bundle can have arbitrary library folders, an activity using compiled libraries must implement code to selecting the right one for the current architecture.

After a few activities have done this, perhaps a convention can be recommended in these docs for others to follow.

(Thanks to Tony Anderson and Bert Freudenberg for clarifying this issue)