build(cert-create): don't generate `build_msg.c`

This change avoids generating a build message source file on the shell,
instead using the `__DATE__` and `__TIME__` macros directly.

Change-Id: Ieda75bbac174847c716701bce8dd10b8e9975902
Signed-off-by: Chris Kay <chris.kay@arm.com>
2 files changed