Bug#860304: flash-kernel: Incorrect installation path for dtbs

[Heinrich Schuchardt <xypron.glpk@gmx.de>]

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