[U-Boot] SAMA5D2 xplained SD/eMMC boot

Hi!

I've been playing around with latest mainline u-boot on sama5d2 xplained ultra. I noticed that if I want to boot the board from SD card (SDHCI1), the board will indeed load the SPL from it, but SPL will try to load u-boot.img from eMMC (SDHCI0) and fail, as my eMMC is blank. This is a result of hard-coding boot device to BOOT_DEVICE_MMC1 in arch/arm/mach-at91/spl.c spl_boot_device() I think.

Is there any way to discern from which SDHCI the board booted to continue loading u-boot.img from the correct one ? This would let us implement board_boot_order() and boot from correct SDHCI.