bsp: Add boot-order to secure partition manifests

Previously, the order in which secure partitions were initialized relied
on the order in which they were loaded - however that is undocumented
behavior.

Add boot order property into SP manifests to ensure each partition is
initialized in the correct order i.e before any other partition which
uses it
- block storage before protected storage
- protected storage before crypto

Signed-off-by: Adam Johnston <adam.johnston@arm.com>