Install static element files

Several elements want to install static files. Some do this using the
"os-apply-config" directory, which means these files are unnecessarily
rewritten every time os-refresh-config triggers. Others put files in a
"files" dir and manually script the installation from an install.d
script.

This patch offers a more convenient "static" directory. Files placed in
here will be rsynced into the image. This assumes all files are meant to
be installed with user and group root.

Co-Authored-By: Gregory Haynes <greg@greghaynes.net>
Change-Id: I390b822bf80ff6470b25a3473326cac362c4497a

elements/install-static/README.md

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+Copy static files into the built image.
+
+The contents of any ``static/`` subdirs of elements will be installed into
+these images at build time using ``rsync -lCr``. So to install a file
+``/etc/boo``, include ``static/etc/boo`` in your element.
+
+Note: This installs all files with owner and group of root.

elements/install-static/install.d/10-install-static-files

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+#!/bin/bash
+# Note that this relies on the detail that all elements share one dir inside
+# the chroot. This will copy all the files that elements have added to
+# element/static into the image. Mode, symlinks etc will be respected.
+set -eux
+
+STATIC_SOURCE="$(dirname $0)/../static"
+[ -d "${STATIC_SOURCE}" ] || exit 0
+
+rsync -lCr "${STATIC_SOURCE}"/ /