mk: Add --enable-local-rebuild to bootstrap from the current release

mk: Add --enable-local-rebuild to bootstrap from the current release

In Linux distributions, it is often necessary to rebuild packages for
cases like applying new patches or linking against new system libraries.
In this scenario, the rustc in the distro build environment may already
match the current release that we're trying to rebuild.  Thus we don't
want to use the prior release's bootstrap key, nor `--cfg stage0` for
the prior unstable features.

The new `configure --enable-local-rebuild` option specifies that we are
rebuilding from the current release.  The current bootstrap key is used
for the local rustc, and current stage1 features are also assumed.