dropbear: fix initscript install

The -D option to install only works portably when the destination is
a file. Specifying the filename avoids the following build error (at
least on some hosts):

"/usr/bin/install: cannot overwrite directory `/tmp/buildroot/
project_build_foobar/uclibc/root/etc/init.d' with non-directory"

Signed-off-by: Marc Gauthier <marc@tensilica.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>