I have debian900 booting from emmc. So it is possible
If i remember correctly the only "problem" was the numbering change depending on whether a sd card is in the device or not.

emmc becomes
/dev/mmcblk0 without sdcard
/dev/mmcblk1 with sdcard in device

I "solved" it via 2 different uboot entrys.
But i guess you need just one.