tools/env: Add environment variable flags support

Currently just validates variable types as decimal, hexidecimal,
boolean, ip address, and mac address.  Call
env_acl_validate_setenv_params() from setenv() in fw_env.c.

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.

Need to build in _ctype for isdigit for Linux.

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