ci: Build more targets, cache SDKs, eatmydata

Build graph-depends and legal-info along with firmware images

Also, use proper O=/foo for telling buildroot where to build images.
Also, use the magic buildroot config script rather than hacky hacks.

Support building and caching an SDK, meaning you can build simple
defconfigs in dozens of minutes, and greatly reduce the total time
for building all configurations.

Eatmydata will save us a *lot* of fsync() activity when you don't
have infinite IOPS.

Signed-off-by: Stewart Smith <stewart@linux.ibm.com>
diff --git a/ci/Dockerfile/ubuntu1604 b/ci/Dockerfile/ubuntu1604
index 46f6801..2aaa647 100644
--- a/ci/Dockerfile/ubuntu1604
+++ b/ci/Dockerfile/ubuntu1604
@@ -7,4 +7,4 @@
   build-essential g++ git bison flex unzip \
   cpio vim-common lsb-release \
   libxml-simple-perl libxml-sax-perl libxml2-dev libxml2-utils xsltproc \
-  wget bc libssl-dev
+  wget bc libssl-dev python-matplotlib python-numpy graphviz eatmydata