Measured boot: Add NS API interface library

Adds an interface library for the measured boot NS APIs, so that users
of the NS APIs have something to link against.

Also reduces scope of some items to PRIVATE where they didn't need to
be public.

Signed-off-by: Jamie Fox <jamie.fox@arm.com>
Change-Id: I11c73c1e2d72bfe2c06ffea6fdab87dd022f5531
1 file changed