fng-install: Fixed post-install for fngsystem

The post-install for fngsystem is running in a temp directory.
This directory was missing the new artifacts (devicetree and overlays)
to properly run.
Further, the combine step of kernel + devicetree was failing for
fngsystem, because the fngsystem partition is too small (at least for
i.MX8 devices). To work around this, the post-install scripts checks
for a "combine_kernel" file in the boot partition and only combines
the kernel and the devicetree if the file is present.

Cherry-pick from 4c3847e5