Allow SRCREV handling through poky-agl.conf

Allow SRCREV handling through poky-agl.conf

This change simplifies the SRCREV handling by using
- AGL_APP_REVISION and AGL_DEFAULT_REVISION
in recipes.
Also the AGL_BRANCH can be used to switch to release branches.

Bug-AGL: SPEC-864

Change-Id: I9384c85d4f7434416d87359f88c482887ac2ead8
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/11685
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>