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/drivers/video/mx3fb.c b/drivers/video/mx3fb.c
index 51d06d6..78e595e 100644
--- a/drivers/video/mx3fb.c
+++ b/drivers/video/mx3fb.c
@@ -816,7 +816,7 @@
 
 	videomode = CONFIG_SYS_DEFAULT_VIDEO_MODE;
 	/* get video mode via environment */
-	penv = getenv("videomode");
+	penv = env_get("videomode");
 	if (penv) {
 		/* decide if it is a string */
 		if (penv[0] <= '9') {