move GPLv2 logic into agl-gplv2 feature

move GPLv2 logic into agl-gplv2 feature

Currently, we hardcode GPLv2/GPLv3 logic directly into the poky-agl
distro config, and in agl-demo. However, the recipes in meta-gplv2 are
very old, poorly supported, and receive no security updates. So, they
can easily cause issues.

To allow the choice of both GPLv2 and GPLv3, refactor the meta-gplv2
logic into its own "agl-gplv2" feature. The default is GPLv3, which
points to well-supported layers with security updates. However, by
adding the "agl-gplv2" feature in aglsetup.h, the meta-gplv2 layer gets
added to bblayers.conf and recipe versions get set to those in
meta-gplv2.

Change-Id: I62d7f2daa58b0690b5b27740257bd15097fcee5c
Signed-off-by: Martin Kelly <mkelly@xevo.com>