docker/coreboot-sdk: clone coreboot submodules when doing test build

Change-Id: I2315beda31bdc8edc92d21b6665eb5ebd07da2e6
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49961
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

util/docker/coreboot-sdk/Dockerfile

@@ -123,6 +123,7 @@ RUN mkdir -p /tmp/work && \
   git clone https://review.coreboot.org/em100.git && \
   make -C em100 && \
   git clone https://review.coreboot.org/coreboot.git && \
+  (cd coreboot && git submodule update --init --checkout ) && \
   make -C coreboot CPUS=$(nproc) test-abuild && \
   \
   make -C coreboot olddefconfig && \