Allow CONFIG_REGEX to be disabled when CONFIG_NET

Instead of selecting REGEX when NET is enabled, make it the default, but
allow boards that are tiny to disable it and lose functionality on all
but the first Ethernet adapter.

cm-bf548, bf538f-ezkit, and bf533-stamp need this. None appear to have
more than one Ethernet interface.

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