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/mb862xx.c b/drivers/video/mb862xx.c
index 1c74e97..e0565e1 100644
--- a/drivers/video/mb862xx.c
+++ b/drivers/video/mb862xx.c
@@ -246,7 +246,8 @@
 	tmp = 0;
 	videomode = 0x310;
 	/* get video mode via environment */
-	if ((penv = getenv ("videomode")) != NULL) {
+	penv = env_get("videomode");
+	if (penv) {
 		/* decide if it is a string */
 		if (penv[0] <= '9') {
 			videomode = (int) simple_strtoul (penv, NULL, 16);