Move generic-arm64 machine template to meta-agl

A machine template for the generic-arm64 machine from meta-arm for ARM SystemReady compatible (i.e. EFI boot) images was added in meta-agl-devel during the parallel next branch development for AWS testing.  Now that kirkstone has been merged to master branch, it would seem to make sense to move the machine template to meta-agl with the other community supported machines.  As well, discussion in the virtualization EG indicates that booting images on Graviton instances in AWS is an ongoing thread of development that will continue, so making the support more obvious and easy to maintain seems worthwhile.