Added some de-facto style guidelines to HACKING and fixed violations

This patch adds several guidelines:

* Global constants should be ALL_CAPS (cfg => CFG)
* Prefer single-quotes over double-quotes ("foo" => 'foo')
* Place a space before TODO in comments ("#TODO" => "# TODO")

Change-Id: Ib5b5c5916744856eca2ecaa37e949a3cdc4b3bd7
Merge "Added some de-facto style guidelines to HACKING and fixed violations"