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>