classes: refactor bbclass buildinfo for phygittag
classes: refactor bbclass buildinfo for phygittag

This patch refactors the bbclass buildinfo to work nicely with
phygittag. The changes are

  - use variable GIT_TAG instead of TAG
  - remove support to checkout BRANCH, only tag is supported anymore.
    This simplifies the code.
  - update examples: show usage with phygittag and without
  - remove variable PV in local.conf snippets. The recipe has to
    use SRCPV in the variable PV or use the bbclass phygittag.
    Setting PV in the local.conf is a bad idea.

All recipes, which defines TAG to use the old buildinfo interface, has
to be updated.

Signed-off-by: Stefan Christ <c04781dbce194802ec6a6311a1200fbcd0141e96@phytec.de>
Signed-off-by: Stefan Müller-Klieser <a3e08e35e4a42be768e7d0e76ae1ba321c2a66dd@phytec.de>

PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec,PHYTEC-Messtechnik-GmbH/meta-phytec