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/board/freescale/mx51evk/mx51evk_video.c b/board/freescale/mx51evk/mx51evk_video.c
index 86ec750..cc2c855 100644
--- a/board/freescale/mx51evk/mx51evk_video.c
+++ b/board/freescale/mx51evk/mx51evk_video.c
@@ -76,7 +76,7 @@
 int board_video_skip(void)
 {
 	int ret;
-	char const *e = getenv("panel");
+	char const *e = env_get("panel");
 
 	if (e) {
 		if (strcmp(e, "claa") == 0) {