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>

env/callback.c

@@ -51,7 +51,7 @@ void env_callback_init(ENTRY *var_entry)
 	int ret = 1;
 
 	if (first_call) {
-		callback_list = getenv(ENV_CALLBACK_VAR);
+		callback_list = env_get(ENV_CALLBACK_VAR);
 		first_call = 0;
 	}