If you have both engines installed but one is broken you are stuck
with the automagic. Allow the user to override the engine for this
case.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20231009164104.369749-7-alex.bennee@linaro.org>