Create a new class distutils-common-base.bbclass which holds the common
	parts that can be used in native and target packages which need to use
	distutils funtionality.

	rpm, libxml2, zope are currently using them and needed to use
	distutils-native-base for native recipes.

	rpm and libxml need to defer the processing of certain configure
	parameters which we evaluate using python. So we need to have python-native
	built before we can process them. Hence we can not use EXTRA_OECONF
	which is a python variable and gets expanded during parsing recipes
	and ofcourse we have not yet built python-native.

	We pass these extra options as a separate shell variable to do_configure
	which we evaluate when that task is executing.