olsr: rework build and install procedure

This commit reworks the build and install steps of the olsr package,
to fix a number of problems and do minor improvements:

 1. The build step was doing "make <foo>" for each
    plugin. Unfortunately, inside olsr build system, doing "make
    <foo>" for a plugin triggers a clean of the plugin directory, a
    build, and then an installation of the plugin. This installation
    fails because DESTDIR is not passed at the install step. This
    leads to build issues like
    http://autobuild.buildroot.org/results/a9fed78cd0b3991763a797c38387edd4157fbdb9/build-end.log

. To
    fix this, we call make in each plugin's directory, which is what
    "make <foo>" was doing internally.

 2. The install step was manually installing the olsrd binary. We now
    instead use the install_bin make target that comes in olsr's build
    system.

 3. The install step was manually installing the olsr plugins. We now
    instead use the install make target available in each plugin's
    makefile.

 4. We use 'install' to install the init script, which avoids the
    manual creation of /etc/init.d.

 5. We use 'install' to install the sample configuration file.

 6. We remove the useless strip commands.

 7. We add a patch that allows us to pass LDCONFIG=/bin/true to avoid
    ldconfig being called during the installation process.

 8. We remove commands from the clean step that were in fact
    uninstallation commands. We don't bother re-adding those commands
    in an uninstallation step, since it is now generally accepted that
    the uninstall step is quite useless and should be deprecated.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>