buildhistory.bbclass: Improve robustness in image file listing
The filenames sometimes may have strange names. With the 'awk' script
it handled a limited number of spaces in the filename and a package
installing a file named "test file with spaces" would have its name
truncated.
This patch uses the find's printf formating to simplify the code and
properly handle this case. From a testing image, the only diff produced
is:
