network-generator: support BOOTIF= and rd.bootif=0 options

network-generator: support BOOTIF= and rd.bootif=0 options

The network generator currently supports many of the options described
by dracut.cmdline(7), but not everything.

This commit adds support for the BOOTIF= option (and the related
rd.bootif= option) used in PXE setups.

This is implemented by treating BOOTIF as a special name/placeholder
when used as an interface name, and expecting a MAC address to be set in
the BOOTIF= parameter. The resulting .network file then uses MACAddress=
in the [Match] section, instead of Name=.