bootloader: Add mx6-boot as image containing fngboot as 1st and uboot as optional 2nd stage.

For edgehog we need u-boot features but for RAM calibration and so on fngboot is a well known solution for SECO Ne imx6 HMIs. To get "the best of both worlds" we combine both, fngboot and u-boot without SPL, to one boot image.

fngboot decides in the first stage if u-boot needs to be booted. This is done when either no FAT partition is found or a seco_boot.scr is found on a fat partition of the boot device. In this case uboot is started from a known offset in the image to handle the further boot process.

If this path is not taken, the normal fngboot procedure from FAT partitions with boot scripts like boot.cfg is used. The RTC based bootselect mechanism still applies here.