bb-matrix: Fix min and max calculations

bb-matrix: Fix min and max calculations

The BB and PM ranges were originally intended to use leading 0s to
ensure all the values were the same string length, making for nice log
filenames and columnar dat files. However, not everyone will do this -
especially if it isn't documented.

Document the intent. Make the generation and parsing of dat files robust
to either method.

(From OE-Core rev: 90dc44c8246f2a580fe4a41ce28c201e52307500)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>