Build: Add option to enable or disable Audit Log

Adds a build option that makes it possible to enable or disable the
TF-M Audit Log partition. The option TFM_PARTITION_AUDIT_LOG can be
provided in the CMake configure command, and is enabled by default if
not supplied.

Change-Id: Ib60f22aeb62652e34a1ac43a61154288e6d989b3
Signed-off-by: Jamie Fox <jamie.fox@arm.com>