install-files: Replace symlinks into linux-bootwrapper package with files

In linux version 4.17.6-1, the bootwrapper executables for PowerPC
architectures were moved to a separate binary package and the
linux-image package contains symlinks to (and a dependency on) that.

The d-i build process needs these files and expects them to be part of
the kernel-image udeb, so replace the symlinks with the target files.