python3-build: simplifly native bootstrap

python3-build: simplifly native bootstrap

The pep517 class no longer called the "build" module directly, so we
can't play games with PYTHONPATH here.

However, the flit_core class has a bootstrap method so this recipe can
use that instead.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>