Fix UEFI PXE boot on a NIC that isn't the first NIC.

In UEFI it appears all available NICS are present to pass network traffic.
This gives the capability to load the loader.efi from disk then set
currdev="net3:" and then all I/O will over over the 2nd NIC. On this
machine is appears the first handle is the first NIC in IPv4 mode and
then the 2nd handle is the first NIC in IPv6 mode. The 3rd handle is
the 2nd NIC in IPv4 mode. The fix is to index into the handle based
on the unit cached from boot device passed into the loader.

Some testing info from a test boot via kenv:
currdev="net3:"
loaddev="net3:"
boot.netif.name="igb1"