rsb(rtems-build-dep): handle multiple library paths

Summary

The script rtems-build-dep might fail to find a library if multiple -L path arguments are passed.

The argument parser only keeps the last library path provided:

# source-builder/sb/rtems-build-dep
            libraries="$2"; shift;
            shift;;

Take the following example. We are trying to find libpython3.so, which can be found on /usr/lib:

$ rtems-build-dep -c gcc -L /usr/lib -l libpython3.so # this works because we only pass -L once
> found

$ rtems-build-dep -c gcc -L /tmp/ -L /usr/lib -l libpython3.so # this also works because /usr/lib is the last path provided
> found

$ rtems-build-dep -c gcc -L /usr/lib -L /tmp/ -l libpython3.so # this fails because /usr/lib is discarded
> not-found

I've made a change to handle multiple library paths in the same fashion multiple include directories are handled.

--

I don't know the implications of this change. So far, I've managed to build the 6/rtems-arm bset successfully using this changes.