yocto/[images/baremetal]: Reduce image size

The total RAM size is 2G and 32M of it has been allocated to OP-TEE. The
remaining RAM space size is (2048 - 32)M, so BAREMETAL_IMAGE_MEM_SIZE
should be updated.

Signed-off-by: Jiamei Xie <jiamei.xie@arm.com>