add support for alternative C libraries (currently only glibc/eglibc) other (related) changes: - kernel headers are now installed using "make headers_install" on 2.6 - target names now contain an openwrt "vendor" tag (e.g. mips-openwrt-linux-gnu) - build directory names now contain gcc/libc name/version - default cpu for x86 is now i486 (required to build glibc/eglibc)

SVN-Revision: 13931