Xilinx ZYNQ supports MMC/eMMC as secondary boot media. This can be enabled by FSBL settings, it is not required to make any changes to the FSBL generated by Xilinx SDK.
Example debug log from MMC boot on TE0720-02 on TE0701, an MMC Card was inserted into SD Card slot. To enable TE0720 to boot from SD Card in the slot, a small piece of paper was used to disable the card detect switch.
In order to change the boot device to SDIO1 a change is needed in diskio.c in the FSBL_bsp:
+++ #define SD_DEVICE_ID XPAR_XSDPS_1_DEVICE_ID