Scons: fix the Win32Spawn

Windows(tm) CreateProcess does not use the env passed to it to find the
executables. So we have to modify our own PATH to make Popen work. Also,
detect the cmd build-in commands and execute it directly.

Tested on MDK toolchain on RB4088 and the MSVS toolchain on simulator
BSP.
Merge pull request #283 from grissiom/win32spawn

Scons: fix the Win32Spawn