pb-event: Add 'boot' user event

Add a user event to send a boot command to the discover server.
The format of the boot command is similar to the add command, eg:

pb-event boot@eth0 image="http://host/image" initrd="http://host/initrd"

and also recognises "id", "dtb", and "args" arguments.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>