buildhistory: allow disabling image and/or package history
Add a BUILDHISTORY_FEATURES variable which can be set to "" to disable
buildhistory with the class still inherited.
BUILDHISTORY_FEATURES by default contains two items - image and package.
You can use these to disable the image and package history functions
individually - this is particularly useful if you want to get the image
contents and dependency graphs but don't need the package history.
Additionally, ensure we quit shell procedures gracefully by using return
instead of exit.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
