loader/efi/fdt: Add fdtdump command to access device tree

loader/efi/fdt: Add fdtdump command to access device tree

The fdtdump command allows dumping arbitrary device tree properties
and saving them to a variable similar to the smbios command.

This is useful in scripts where further actions such as selecting
a kernel or loading another device tree depend on the compatible
or model values of the device tree provided by the firmware.

For now only the root level properties of the dtb are exposed.

Signed-off-by: Tobias Heider <tobias.heider@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>