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/include/net.h b/include/net.h
index a6f9082..455b48f 100644
--- a/include/net.h
+++ b/include/net.h
@@ -834,7 +834,7 @@
 ushort string_to_vlan(const char *s);
 
 /* read a VLAN id from an environment variable */
-ushort getenv_vlan(char *);
+ushort env_get_vlan(char *);
 
 /* copy a filename (allow for "..." notation, limit length) */
 void copy_filename(char *dst, const char *src, int size);