[CMake] set CMAKE_TOOLCHAIN_FILE inside cmake file

[Jun Sun]

I am using the 2.6 branch. Very nice to see crosscompile working well.
Thanks.

One question is that CMAKE_TOOLCHAIN_FILE seems only working when set
at commandline, but not from inside CMakeList.txt, true?

Basically I generate some config.cmake file, and then was hoping
to select the toolchain file based on some variables defined in 
config.cmake file.

Cheers.

Jun