add /usr/local flags to allow compile with NFS shared XBian rootfs

This commit
- removes some cruft
- fixes the /opt/vc include stuff so links to /usr/{lib,include} aren't needed
- add ${SDKSTAGE}/usr/local CFLAGS, LDFLAGS, RLINK_PATH and PKG_CONFIG so it's possible to use an NFS shared XBian install as bcm-rootfs (with xbian-package-cec etc..)

Oh, and your xbian buildfs on sf.net could really need an update :)