sysutils/opensbi: update to version v1.0

As the fu540 platform has been removed with this version, fall back to
building the generic platform only. Keep the platform logic around in
the Makefile in case we need it in the future.

Switch to building with base system clang+binutils.

Bump the PORTREVISION for sysutils/u-boot-sifive-fu540.

Reviewed by: jrtc27, lwhsu (previous version both)
Approved by: lwhsu (ports)
Tested by: Klaus Küchemann <maciphone2@googlemail.com>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33845