bitbake.conf/classes: Variable cleanup

bitbake.conf/classes: Variable cleanup

This patch removes the variables BASE_PACKAGE_ARCH, BASEPKG_HOST_SYS,
BASEPKG_TARGET_SYS and also removes the immediate assignments in
several core classes as these are no longer required.

This should make it clearer what some of the core variables do and
simplfy some overly complex and confusing class code.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>