README: fix meta-efl, meta-gpe and meta-xfce to point to openembedded.org

Fix README files for meta-efl, meta-gpe and meta-xfce to point to the correct
openembedded.org address instead of the outdated openembedded.net in git URIs.
Reported by Autif Khan for one of the layers.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>