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/mxsfb.c b/drivers/video/mxsfb.c
index 0ddce3d..9d810ba 100644
--- a/drivers/video/mxsfb.c
+++ b/drivers/video/mxsfb.c
@@ -161,7 +161,7 @@
 	puts("Video: ");
 
 	/* Suck display configuration from "videomode" variable */
-	penv = getenv("videomode");
+	penv = env_get("videomode");
 	if (!penv) {
 		puts("MXSFB: 'videomode' variable not set!\n");
 		return NULL;