If you really want to boot from an SD card, try putting it in a standard USB card reader. Booting from USB mass storage has been supported for over a decade in BIOS, well before UEFI.
Otherwise, if you want to actually integrate a driver for an internal SD card reader (which isn't a USB one) into the BIOS, then an Option ROM (INT 13H hook) would be the way to go.
Otherwise, if you want to actually integrate a driver for an internal SD card reader (which isn't a USB one) into the BIOS, then an Option ROM (INT 13H hook) would be the way to go.