buildhistory_analysis: improve field handling robustness
Avoid errors when comparing changes for KEY = value files (package info
files and image-info.txt):
* Handle keys appearing and disappearing - this will help to handle PE
in package info files (which is only written when it is not blank) and
when we add additional fields in future.
* Handle when old value is 0 for numeric field (avoid division by zero)
* Report when numeric field was empty or missing rather than 0 (but
still treat it as 0 for comparison purposes)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
