imx8: output SECO-FW and ATF commit IDs

Borrow ID reading code from Ye Li (NXP U-Boot, commit ID 5b443e3e2617)
but drop imx-mkimage commit ID reading since we now use in tree mkimage.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>