--disable-bootstrap changed semantics

[Mike Hommey]

Hi,

I know a few people are using `ac_add_options --disable-bootstrap` to
avoid the regular toolchain updates when they update their tree, and
rely on running `mach bootstrap` on their own schedule.

Making things work for a ton of different use cases is difficult, but
we've reached a different compromise in how this all works, with the
consequence that `--disable-bootstrap` now not only does that but it
also prevents the toolchains in ~/.mozbuild from being used at all.

However, there is a replacement for the previous behavior:
`ac_add_options --enable-bootstrap=no-update`

For the record, these changes happened in bug 1853271, which also
unlocks using the toolchains in ~/.mozbuild on branches such as beta or
release, once it propagates through the train. (so no more need to
manually --enable-bootstrap)

Cheers,

Mike