Add installer factory

Adds an interface for constructing installers. Allows for alternative
sets of installers to be used on different platforms. At the moment,
supported installers are general purpose. It is anticipated that
domain specific installers will be needed with image format specific
knowledge.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: I359561a8d908bac5ba826ad38ba27291a4f06f2d
7 files changed