util/docker: Add bc to coreboot-sdk

bc is one of the standard posix utilities.  I'm surprised that it's
not in the debian docker image by default.

Change-Id: I02f2d5296e7f87876b236af119965d1f4e6a0bc0
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/21889
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/docker/coreboot-sdk/Dockerfile

@@ -19,6 +19,7 @@ RUN \
 	useradd -p locked -m coreboot && \
 	apt-get -qq update && \
 	apt-get -qqy install \
+		bc \
 		bison \
 		bzip2 \
 		ccache \