meta-cassini-bsp: Always build the utilities image for corstone1000-mps3

As the utilities overlay image is always required when using any
corstone1000-mps3 based images in a lava farm and not just when running
tests so we should always build it when the corstone1000-mps3 machine
is used.

Changelog: bug
Signed-off-by: Drew Reed <Drew.Reed@arm.com>