We are now using an env_ prefix for environment functions. Rename these
two functions for consistency. Also add function comments in common.h.

Quite a few places use getenv() in a condition context, provoking a
warning from checkpatch. These are fixed up in this patch also.
Suggested-by: NWolfgang Denk <wd@denx.de>
Signed-off-by: NSimon Glass <sjg@chromium.org>