posinstall:gf_prepare_kernel.sh: Fix non-overlay case

If the overlay created by the gfxml2tdo tool cannot be applied the
`devicetree.dtb` will not be created. In this case the raw platform
device tree should be appended to the kernel.

However, there was a mix-up in the paths and the platform dtb was not
found. Fix this by moving the $MOUNTPOINT variable to the cat command
and remove it from the $DEVICETREE_APPEND_FILE in case of a successful
overlay.

BCS 746-000150