boot CoreELEC from eMMC and run this commands to allows booting Batocera from USB stick
fw_setenv start_usb_autoscript 'for usbdev in 0 1 2 3; do if fatload usb ${usbdev} ${loadaddr} s905_autoscript; then autoscr ${loadaddr}; fi; done;'
fw_setenv start_mmc_autoscript 'if fatload mmc 0 ${loadaddr} s905_autoscript; then autoscr ${loadaddr}; fi;'
fw_setenv bootfromusb 'usb start 0; run cfgloadusb; run start_usb_autoscript; if fatload usb 0 ${loadaddr} kernel.img; then run usbdtb; setenv bootargs ${bootargs} bootfromusb; bootm; fi'
fw_setenv bootfromsd 'if mmcinfo; then run cfgloadsd; run start_mmc_autoscript; if fatload mmc 0 ${loadaddr} kernel.img; then run sddtb; setenv bootargs ${bootargs} bootfromsd; bootm; fi; fi'
create Batocera USB/SD
download archive BATOCERA.ZIP and inside is file u-boot.ext (of course this file is probably only for GT-King Pro)
copy this file to your Batocera USB/SD
device will boot CoreELEC from eMMC without USB/SD attached and from USB/SD with Batocera or CoreELEC
Maybe I made some typo above. If it doesn’t work let me know.
Maybe would be just enough to copy s905_autoscript to cfgload and then it would boot without any modifications anywhere. Should test on virgin CE device …