classes/buildhistory: track contents of selected files in images

classes/buildhistory: track contents of selected files in images

Add a BUILDHISTORY_IMAGE_FILES variable specifying a space-separated
list of files within an image to copy into buildhistory, so that changes
to them can be tracked. Typically this would be used for configuration
files, and by default this includes /etc/passwd and /etc/group, but the
user is free to extend this list by setting the variable in local.conf.

Implements [YOCTO #4154].

(From OE-Core rev: ed6bb6e3db518082d3a9c45d548bc1339be2c5ca)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>