Pull request for UEFI sub-system for efi-2021-04-rc1
In the UEFI sub-system:
* implement non-blocking file services
* print boot device and file path in helloworld.efi
* improve detection of boot device
* correct argument handling in efivar.py
* implement EFI_DT_FIXUP_PROTOCOL
Bug fixes:
* adjust conitrace command for low baud rates
* check that FIT images are valid FDTs