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.