base.bbclass: Fix PR increment bug when PR number is a single digit
PRINC which should add to base PR value has a problem when
the PR is single digit e.g. r0 - r9. Current algorithm
needed atleasts 2 digits to successfully populate end and begin
markers.
We reimplement the incrementing algorithm using regular expressions
which addressed the above mentioned problem and
simplifies the logic a bit and gets rid of loops and conditionals
