efi_selftest: use GUID to find ESP in dtbdump

If dtbdump.efi is loaded from memory when calling LoadImage the loaded
image protocol will not indicate the partition from where it was loaded.
In this case use the EFI system partition for the 'load' and 'save'
commands.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>