test: check for binariers in [usr/][s]bin

test: check for binariers in [usr/][s]bin

The image build function greps for ExecStart lines in unit files, but some
of them (eg: systemd-firstboot) do not use a full path.
It then falls back to 'type -P' but that only works if you have the binary
installed. For optional binaries like systemd-firstboot, the installation
can then fail.

Manually check if the binary already exists in /[usr/][s]bin.