env: Rename some other getenv()-related functions

We are now using an env_ prefix for environment functions. Rename these
other functions as well, for consistency:

   getenv_vlan()
   getenv_bootm_size()
   getenv_bootm_low()
   getenv_bootm_mapsize()
   env_get_default()

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

net/net.c

@@ -1536,7 +1536,7 @@ ushort string_to_vlan(const char *s)
 	return htons(id);
 }
 
-ushort getenv_vlan(char *var)
+ushort env_get_vlan(char *var)
 {
 	return string_to_vlan(env_get(var));
 }