buildhistory: track contents of selected files in images

:Release Notes:
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.

:Detailed Notes:
Backport of oe-core commit ed6bb6e3db518082d3a9c45d548bc1339be2c5ca

Implements [YOCTO #4154].

:Testing Performed:
Build tested, buildhistory output checked.

:QA Notes:
No change in image

:Issues Addressed:
[GF-3127] Backport newer buildhistory.bbclass
[GF-9902] Disable checking for package version decreases

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>

Change-Id: I39950cda78ca2aad7cc3fe204e495225502153ea
Reviewed-on: https://g2g.palm.com/3046
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
Tested-by: Susan Montooth <susan.montooth@lge.com>