env: Rename getenv_hex(), getenv_yesno(), getenv_ulong()

We are now using an env_ prefix for environment functions. Rename these
for consistency. Also add function comments in common.h.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>

net/net.c

@@ -329,7 +329,7 @@ void net_auto_load(void)
 		return;
 	}
 #endif
-	if (getenv_yesno("autoload") == 0) {
+	if (env_get_yesno("autoload") == 0) {
 		/*
 		 * Just use BOOTP/RARP to configure system;
 		 * Do not use TFTP to load the bootfile.