ci: Move everything to a separate directory

We're going to have a few more CI-related files in a second, and
it makes sense to have a separate directory for them rather than
littering the root directory.

$(CI_SCRATCHDIR) can now also be created inside the CI directory,
and as a bonus the make rune necessary to start CI builds without
running configure first becomes shorter.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

.gitignore

@@ -44,7 +44,7 @@
 /autom4te.cache
 /build-aux/*
 /build/
-/ci-tree/
+/ci/scratch/
 /confdefs.h
 /config.cache
 /config.guess