cbfs: Move more stuff into cbfs_boot_lookup()

cbfs_boot_locate() is supposed to be deprecated eventually, after slowly
migrating all APIs to bypass it. That means common features (like
RO-fallback or measurement) need to be moved to the new
cbfs_boot_lookup().

Also export the function externally. Since it is a low-level API and
most code should use the higher-level loading or mapping functions
instead, put it into a new <cbfs_private.h> to raise the mental barrier
for using this API (this will make more sense once cbfs_boot_locate() is
removed from <cbfs.h>).

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: I4bc9b7cbc42a4211d806a3e3389abab7f589a25a
---
M src/include/boot_device.h
M src/include/cbfs.h
A src/include/cbfs_private.h
M src/lib/cbfs.c
M src/security/tpm/tspi/crtm.c
M src/security/tpm/tspi/crtm.h
6 files changed, 74 insertions(+), 41 deletions(-)