env: Rename getenv/_f() to env_get()

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: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/itest.c b/cmd/itest.c
index e1896d9..70db04a 100644
--- a/cmd/itest.c
+++ b/cmd/itest.c
@@ -101,7 +101,7 @@
 			i++;
 		}
 		s[i] = 0;
-		return  getenv((const char *)&s[2]);
+		return  env_get((const char *)&s[2]);
 	} else {
 		return s;
 	}