Update makefile to detect Raspberry Pi / Raspbian

Previous method returned hostname, which could easily be different from "raspberrypi" if changed by the user, suggested change relies on looking in /etc/os-release for ID=raspbian which is hostname agnostic.
Fixed to prevent error causing false positive

Returned string on missing file was non-empty, thus causing false detection of Pi. Fix tested and working on Mac OS Yosemite.
Merge https://github.com/Gadgetoid/PropellerIDE