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>

common/bootretry.c

@@ -23,7 +23,7 @@ static int      retry_time = -1; /* -1 so can call readline before main_loop */
  */
 void bootretry_init_cmd_timeout(void)
 {
-	char *s = getenv("bootretry");
+	char *s = env_get("bootretry");
 
 	if (s != NULL)
 		retry_time = (int)simple_strtol(s, NULL, 10);