gitpkgv: add support SRCREV_FORMAT

In case of multiple GIT repositories are used, SRCREV_FORMAT will be
respected while filling it with the proper GIT information to each
revision.

This new feature needed heavy changes in the code so basically it was
a rewrote version that keeps compatibility with previous usage.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>