bootstrap: check in the merge-sections script.

We need build-aux/merge-sections available at bootstrap time so
that bootstrap can rebuild itself to check for updates.
* bootstrap.conf (require_bootstrap_uptodate): Improve failure
message.
* .gitignore: Remove build-aux/merge-sections.
* build-aux/merge-sections: Check in.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>