Makefile: fix kernel modules test

Commit fde26057 (Makefile: test before search for kernel modules)
changed the way we strip kernel modules, but it fails when modules aren't
available (as test -d returns with a non zero exit code).

Fix it by including the test -d call in a proper shell conditional.

Reported-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>