Makefile: don't override RSYNC_VCS_EXCLUSIONS

If some packages need to exclude more patterns during the rsync build
step the RSYNC_VCS_EXCLUSIONS can't be redefined here.
webkitgtk: don't rsync LayoutTests

This is a waste of space at this point.
webkitgtk: proper Debug/Release build parameters
rpi_webkitgtk_defconfig: enable Avahi
rpi_webkitgtk_defconfig: enable shared-mime-info