[netdevice] Add "linktype" setting

[netdevice] Add "linktype" setting

Add a new setting to provide access to the link layer protocol type
from scripts.  This can be useful in order to skip configuring
interfaces based on their link layer protocol or, conversely,
configure only selected interface types (Ethernet, IPoIB, etc.)

Example script:

    set idx:int32 0
    :loop
    isset ${net${idx}/mac} || exit 0
    iseq ${net${idx}/linktype} IPoIB && goto try_next ||
    autoboot net${idx} ||
    :try_next
    inc idx && goto loop

Signed-off-by: Pavel Krotkiy <porsh@nebius.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>