Additional platform helpers required

[Richard Huddie]

New helper methods are required to enable more advanced device/platform status to be queried.

Some new methods have been added to:
https://bazaar.launchpad.net/~canonical-platform-qa/ubuntu-sanity-tests/trunk/view/head:/ubuntu_sanity_tests/helpers/autopilot.py

These allow detection of platform channel name (devel/rtm), device name and agps support.

But additional helpers should also be added to include things like memory card and SIM card status. (plus anything else useful). This would also need to support desktop use.

The current autopilot.platform.image_codename() and autopilot.platform.model() methods don't always return useful results.