Try gcc compiler optimization -Oz instead of -Os

[ChristianMiddendorf]

I would like to find out if optimizations with the modern gcc parameters are useful. For the "tiny" images with the optimization parameter -Oz instead of -Os it could save space and help to provide space for community specific customizations.

Unfortunately, the entries in the rules.mk file in the openwrt directory seem to be ignored.

Have you ever tried it or discussed it? If so, where should the parameters be stored and in what form?