netroot: Add ip= cmdline checker

netroot: Add ip= cmdline checker

As with other netroot boot arguments ip= lines should be parsed and
checked. Having this has the advantage that other cmdline scripts
can tell the ip= parser if dhcp or ip=:srv:... is needed, like
parse-root.sh (renamed dhcp-fallback) or parse-nfsroot do.

The nfs test-suite has one check which uses static ip lines, but
the autoconf param is wrong. Fixed this as well.