trusty:keybox: only build the fake keybox if configured

The fake keybox offers no security and serves on as a sample as to where
a true hwcrypto implementation may be implemented.  Only build it if it
is configured.  Also rename the implementation for clarity.

Bug: 323831890
Test: build.py
Change-Id: I60cd5511861b5b5acf62470e5ce1961ecd49a957
3 files changed