Makefile: test before search for kernel modules

The 'find $(TARGET_DIR)/lib/modules' used to find and strip kernel
modules fails when no kernel modules have been installed. While the
'|| true' prevents the entire build from failing, there are still some
error messages displayed, which is not nice.

Instead, test if the directory exists before doing the find. We also
remove the '|| true' in order to really abort the build if a
problematic error occurs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>