yocto/baremetal-image: Specify number of CPUs for BSP test case

The BSP SMP test case uses the device tree to determine the number of
CPUs to validate. However, baremetal-image is overriding this by
specifiying maxcpus on the Linux command line. Override the number of
CPUs validated by the test case by specifiying TEST_SMP_NUM_CPUS in the
image recipe.

Additionally factor out the number of CPUs to the variable
BAREMETAL_IMAGE_NUM_CPUS so that it can be changed in one place.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>