Parallel build support for CMake

Enables (hack, probably) to force CMake generated Makefile to perform parallel build. This should speed up the compile process significantly.

Summary:

• Adds MAKEFLAGS=-j%MAKE_JOBS% to makex command so CMake would respect them.
• Both cmd.exe and Powershell version is fixed.

Known issues:

• Doesnt work with *cd targets. (eg. makex bootcd) CMake seems to ignore the flag.