[ipxe-devel] [ipxe/ipxe] file:/ URIs for USB key after loading native USB drivers (#123)

[Michael Brown]

### Description

As a user, I want to be able to use the `file:/` URI syntax to refer to local files on a USB key from which iPXE was loaded even if iPXE includes native USB host controller drivers, since I expect this to work regardless of which drivers are included in the build.

Using the `file:/` URI syntax to refer to local files on a USB key from which iPXE was loaded will currently fail if the iPXE binary includes native USB host controller drivers, since iPXE's loaded image device handle will cease to be valid when iPXE disconnects the platform host controller drivers.

There is no way to prevent the loaded image's device handle from becoming invalid, so an alternative matching strategy will be required.  Recording the device path at startup and matching on device path may be a viable approach.

It may also be worth identifying other uses of `efi_loaded_image->DeviceHandle` to see if similar workarounds are required there.

### Acceptance criteria

* [ ] Ability to boot `bin-x86_64-efi/ecm.usb` (or other iPXE binary with native USB host controller drivers) from a USB key and to subsequently execute a script held on the USB key using the `file:/` URI syntax (with absolute path and no volume label, e.g. `chain file:/script.ipxe`)
* [ ] Other uses of `efi_loaded_image->DeviceHandle` have been identified and triaged

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