+ Update build method on build system to usw BuildType enum.
+ Set CMake variable REZ_BUILD_TYPE based on value of BuildType enum.
+ Set CENTRAL CMake variable if in backwards compatibility mode and BuildType is central.
+ Set REZ_IN_REZ_RELEASE environment variable if in backwards compatibility mode and BuildType is central.
+ Update Doxygen macro to use new REZ_BUILD_TYPE CMake variable.
adding missing arg in call to _add_build_actions
Merge remote-tracking branch 'mark/backwards_compatibility_for_central_variable' into 2.0

Conflicts:
	src/rez/build_process.py
	src/rezplugins/build_system/bez.py