efi_loader: print device-tree in dtbdump.efi

The dtbdump.efi binary can be used for testing the EFI_DT_FIXUP_PROTOCOL.
It provides a command to load a file and have it fixed up and a
command to save the resulting file.

Add a command 'dump' for displaying the device-tree.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>