[ARM] Use bootcmd attribute of /choosen node to populate pre-configured guests

With this patch, all our device tree are divided into five parts:
1. /vmm - generic hypervisor configuration
2. /choosen - selected options
3. /host - host hardware descriptions
4. /guests -  guest descriptions
5. /templates - guest description templates

We pre-include templates of allowable guest types on a host under /templates.

We also use bootcmd to dynamically create guest at boot time.

For now, we assume that guest images will be pre-loaded at boot time but in future once we have MMC/Flash access on boards we can load guest images from bootcmd itself.

This patch also gives a glimpse of dynamic guest creation via bootcmd attribute.

Signed-off-by: Anup Patel <anup@brainfault.org>