cmake: extend CMAKE_MODULE_PATH instead of setting

cmake: extend CMAKE_MODULE_PATH instead of setting

Some (e.g. piglit) CMakeList.txt files will extend CMAKE_MODULE_PATH before
calling project(), which is when the toolchain.cmake file is parsed.  In this
situation the CMAKE_MODULE_PATH is overwritten, so handle this by appending in
toolchain.cmake instead of assigning.

(From OE-Core rev: 2cfa8427d77f680df37c12d00125501ebe7c38a3)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>