[efi] Use current boot option as a fallback for obtaining the boot URI

[efi] Use current boot option as a fallback for obtaining the boot URI

Some systems (observed with a Lenovo X1) fail to populate the loaded
image device path with a Uri() component when performing a UEFI HTTP
boot, instead creating a broken loaded image device path that
represents a DHCP+TFTP boot that has not actually taken place.

If no URI is found within the loaded image device path, then fall back
to looking for a URI within the current boot option.

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