Add blob type for DT overlay according to the update of Firmware Handoff spec[1]. Add an inline header to represent the 'subtype' in a DT overlay blob payload. [1] Add Transfer Entry for Devicetree Overlay https://github.com/FirmwareHandoff/firmware_handoff/pull/74 Signed-off-by: Raymond Mao <raymond.mao@linaro.org> Reviewed-by: Tom Rini <trini@konsulko.com> Tested-by: Michal Simek <michal.simek@amd.com>