Build: add default core number to the build-script

The original default threads num is 1 if we don't type anything here.

I don't know whether this command "cat /proc/cpuinfo | grep "processor" | wc -l" will fit a non-linux OS(e.g. Darwin).
So leave it to 2.

Change-Id: I53dcad2c1a7e6b2c6265d9c8e55864e3c584cbe7
build: use the output directory actually in use.

It is not a good idea to output in the same directory as the source.
Someone may find it hard to manage or clean those files,
others may have two or more hard disks and want to reduce compile time.

We should move other output files(such as OTA and FTP files) to a separate directory in the future.

Change-Id: Ib31f94154938f9ada2d0e392b73db5ed61fb5c5b
Merge changes Ib31f9415,I53dcad2c into kk_mkt

* changes:
  build: use the output directory actually in use.
  Build: add default core number to the build-script