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>

env/common.c

@@ -44,7 +44,7 @@ int env_get_yesno(const char *var)
 /*
  * Look up the variable from the default environment
  */
-char *getenv_default(const char *name)
+char *env_get_default(const char *name)
 {
 	char *ret_val;
 	unsigned long really_valid = gd->env_valid;