Before this change, uninstalling the package would complain about
/tmp/sim_change_start not existing:
root@GL-E750:/tmp# opkg remove blue-merle
Removing package blue-merle from root...
The /tmp/ directory exists.
The /etc/ directory exists.
killall: gltertf: no process killed
No file found within /tmp/tertf. No shredding to be done there.
No file found within /etc/tertf. No shredding to be done there.
Looks like /tmp/ is clean!
Looks like /etc/ is clean!
rm: can't remove '/tmp/sim_change_start': No such file or directory
rm: can't remove '/tmp/sim_change_switch': No such file or directory
No packages removed.
Collected errors:
* pkg_run_script: package "blue-merle" postrm script returned status 1.
root@GL-E750:/tmp#
So we use -f to not raise an error if the file does not exist.
We assume that the version 3 series is compatible, but still prompt the
user if they are using a version that we have not yet tested.
We know that version 4 is not compatible, so we bail out directly.
The double-dollar is for escaping the Makefile.
