(core) Add compatibility patch for future API change
(feature) Remove Xronos from Orcc build

Xronos is now licensed under GPL, but Orcc is still under BSD. These
licences are not compatible, so Orcc releases can't contains Xronos
anymore.
Update all files for 2.1.2 release
Merge branch '2.1.2_forward_compatibility_patch'

Conflicts:
	CHANGELOG.md
	eclipse/plugins/net.sf.orcc.cal.tests/META-INF/MANIFEST.MF