Use $PARALLELISM for make builds to spawn multiple jobs. This results in
better utilization of multi-core processor in the host machine used for
build resulting in lesser build time.

Change-Id: I2986a0ba3baf1db7409f55c018a4ebbcf6ccbc73
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>