Build depends must refer to the source package name, not the binary one,
therefore we need to use `bzip2` in order to enforce a build of libbz2.so.

Also make the host python build depend on the bzip2 host build in order to
ensure that host python is built with bzip2 support. We don't need a build
dependency for the target python as this already depends on +libbz2 through
python-light. That package dependency in turn implies a build dependency.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>