Bug 1693725 - Make --enable-bootstrap install missing toolchains....

Bug 1693725 - Make --enable-bootstrap install missing toolchains. r=firefox-build-system-reviewers,andi,sheehan,mhentges

This means --enable-bootstrap now is the same as what
--enable-bootstrap=install currently does. --enable-bootstrap=install is at the
same time desupported.
We also remove --enable-bootstrap=update, which is not worth supporting.

Differential Revision: https://phabricator.services.mozilla.com/D105719