config: Add support for xconfig

It involved:

 * Modifying the top-level Makefile to create the xconfig target, and
   the $(CONFIG)/qconf target to compile the qconf utility

 * Modifying the package/config/Makefile magic so that C++ files and
   binaries gets linked properly, re-using the
   package/config/Makefile.kconfig logic inherited from the kernel

 * Hacking a little bit package/config/Makefile.kconfig in a way
   similar to the modifications done for the mconf (menuconfig
   interface)

Signed-off-by: Alper Yildirim <ayildirim@aselsan.com>