Add driver library network constructor from file

When a network instance is created with the driver library, the data for
the network often comes from a file. To make this case easier, the
driver library now has a constructor that supports creating the network
directly from the filename.

Change-Id: Id18486491d66076b02e8b78c8ce9933543ced27d
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>