buildhistory: write the contents of the sysroot

Changes to the sysroot are just as interesting during development, so write the
file listing for the sysroot to buildhistory too.

(From OE-Core rev: b3ac82a27ab70ed6996fe3087a578ac637820329)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>