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>

include/environment.h

@@ -282,7 +282,7 @@ extern struct hsearch_data env_htab;
 void env_crc_update(void);
 
 /* Look up the variable from the default environment */
-char *getenv_default(const char *name);
+char *env_get_default(const char *name);
 
 /* [re]set to the default environment */
 void set_default_env(const char *s);