In https://patchwork.ozlabs.org/patch/753871/ I suggested to change U-Boot environment variable fdtfile to meson-gxbb-odroidc2.dtb for the Odroid C2.
Unfortunately this patch was not accepted. So we will need flash-kernel to accommodate U-Boot having a directory path in fdtfile. For the Odroid C2 this is: fdtfile=amlogic/meson-gxbb-odroidc2.dtb Kindly review the patch to flash-kernel I have proposed: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=860304#20 Best regards Heinrich Schuchardt
