[autoboot] Include VLAN tag in filter for identifying autoboot device

[autoboot] Include VLAN tag in filter for identifying autoboot device

When chainloading iPXE from a VLAN device, the MAC address of the
loaded image's device handle will match the MAC address of the trunk
device created by iPXE, and the autoboot process will then erroneously
consider the trunk device to be an autoboot device.

Fix by recording the VLAN tag along with the MAC address, and treating
the VLAN tag as part of the filter used to match the MAC address
against candidate network devices.

Signed-off-by: Michael Brown <mcb30@ipxe.org>