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_

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>

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