env: Add environment variable flags

Currently just validates variable types as decimal, hexidecimal,
boolean, ip address, and mac address.

If the entry is not found in the env ".flags", then look in the static
one. This allows the env to override the static definitions, but prevents
the need to have every definition in the environment distracting you.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>