config/path: add support to use optionally DEBUG=yes as make parameter for...

config/path: add support to use optionally DEBUG=yes as make parameter for building with debugging symbols (for single packages or the whole distro), add support to use optionally DEVTOOLS=yes as make parameter for release builds, setup DEVTOOLS=yes as default on devel builds

Signed-off-by: Stephan Raue <stephan@openelec.tv>