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>
diff --git a/env/flags.c b/env/flags.c
index 85a344f..4b0ddb6 100644
--- a/env/flags.c
+++ b/env/flags.c
@@ -541,7 +541,7 @@
 			return 1;
 		} else if (item->flags &
 		    ENV_FLAGS_VARACCESS_PREVENT_NONDEF_OVERWR) {
-			const char *defval = getenv_default(name);
+			const char *defval = env_get_default(name);
 
 			if (defval == NULL)
 				defval = "";