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.

Change-Id: I61276a1fc71f0bdfd90134b000100e5e64b1ff75
Merge "Install static element files"