envsetup: mka improvements

Use make -j -l y instead of just make -j x
x is threads, y is 2*threads
This way make builds with the maximum number of jobs it can
while not getting the load average above y
I've tested it with 8 (x on my pc), 16, 20 and 24, and
2*threads seemed to be the most efficient

*needs more testing*

Change-Id: Ifd72e699a6fadc08442472defcc764fb91179355