[efi] Add EFI headers from the EFI Development Kit (edk2)
The intention is to include near-verbatim copies of the EFI headers
required by gPXE. This is achieved using the import.pl script in
src/include/gpxe/efi.
Note that import.pl will modify any #include lines in each imported
header to reflect its new location within the gPXE tree. It will also
tidy up the file by removing carriage return characters and trailing
whitespace.