help2man: rewrite recipe

help2man: rewrite recipe

This recipe was originally explicitly minimal because it was needed to
bootstrap autotools, but that isn't the case anymore.

Instead of being explicitly native, use BBCLASSEXTEND.

No need to explicitly depend on autoconf-native automake-native, because
the autotools class does that.

As this recipe isn't needed in early build anymore there's no need to
avoid reconfiguring.

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