busybox

Fix install with hardlinks and a custom PREFIX. Closes 10801

Trying to install busybox with hardlinks and a custom PREFIX will fail
for applets not in the /bin directory, because relative pathnames are
used. applets/install.sh is *supposed to* use the absolute pathname
for hardlinks but it fails to do so because the wrong check is used
in the if statement.

While fixing it, shore up other sloppy coding in applets/install.sh

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>