setuptools3-base: improve RDEPENDS assignment

setuptools3-base: improve RDEPENDS assignment

Use :append instead of += so that recipes can assign to RDEPENDS:${PN}.
This fixes missing RDEPENDS for sixteen recipes in oe-core alone.

Also instead of using obscure inline Python we can just use the
class-target override.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>