Calculate 2 threads per CPU core for build

This will use 2 threads per core, rather than just one.
Should help speed up the build a little.

Take into account Im building on:

Alienware M14x (laptop)
Intel Core i7 4700MQ Quad-Core 2.44GHz CPU
16GB RAM
1TB 7200RPM SATA3 SSHD
Ubuntu 12.04 (ext4)
Dedicated source partition (ext4)

From my last 2 hours of testing, here are my results:

Before change:
Clobber -> Complete      8m 17s

After change:
Clobber -> Complete      6m 39s

Those were calculated averages from each set of runs.

Change-Id: I3ba0b36a909e02239da7f35d49e7b431c0f4c8aa