efi_selftest: add tool to download dtb

For validating the fixed up device tree we need a tool to need to save it
to a file.

dtbdump.efi copies the configuration table with the device tree to a file
on the same partition from which dtbdump.efi was invoked. The file name can
be entered on the console.

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