envsetup: mka: improve automagic job handling
1) multiply nr of cpu cores by 2 to reach optimal cpu load
2) if the user specifies a job value, use that.
3) update instructions

Change-Id: I1fd8ea84a4ee5ce3a484b93b65d0bc08b40a5fce
Signed-off-by: Dennis Rassmann <showp1984@gmail.com>