circleci: provide prereq output in the build log

Commit 200f2666fb1c ("sdk: fix GCC and Python dangling symlinks") in
master tree has forced prereq build step, which is now failing on the CI
as the Docker CI images are missing Python3 support, so now every build
fails with the following error:

 make: *** [staging_dir/host/.prereq-build] Error 1

which is not helpful due to the silenced output of `make defconfig >
/dev/null`, so let's add `make prereq` build step in order to get
something usable next time.

Reported-by: Karl Palsson <karlp@etactica.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

.circleci/config.yml

@@ -94,6 +94,11 @@ jobs:
           working_directory: ~/build_dir
           command: |
              tar Jxf ~/sdk/$SDK_FILE --strip=1
+             touch .config
+             make prepare-tmpinfo scripts/config/conf
+             ./scripts/config/conf --defconfig=.config Config.in
+             make prereq
+             rm .config
              cat > feeds.conf <<EOF
              src-git base https://github.com/openwrt/openwrt.git;$BRANCH
              src-link packages $HOME/openwrt_packages