Fix arm linux toolchain file

Use find_program() to set CMAKE_C_COMPILER and CMAKE_CXX_COMPILER.

Originally the variables was set to compiler name, without absolute path.
CMake 3.0 tries to find them in CMAKE_FIND_ROOT_PATH (since 
CMAKE_FIND_ROOT_PATH_MODE_PROGRAM was set to ONLY) and fails.

platforms/linux/arm-gnueabi.toolchain.cmake

@@ -5,13 +5,12 @@ set(CMAKE_SYSTEM_PROCESSOR arm)
 set(GCC_COMPILER_VERSION "4.6" CACHE STRING "GCC Compiler version")
 
 set(FLOAT_ABI_SUFFIX "")
-
 if (NOT SOFTFP)
   set(FLOAT_ABI_SUFFIX "hf")
 endif()
 
-set(CMAKE_C_COMPILER    arm-linux-gnueabi${FLOAT_ABI_SUFFIX}-gcc-${GCC_COMPILER_VERSION})
-set(CMAKE_CXX_COMPILER  arm-linux-gnueabi${FLOAT_ABI_SUFFIX}-g++-${GCC_COMPILER_VERSION})
+find_program(CMAKE_C_COMPILER NAMES arm-linux-gnueabi${FLOAT_ABI_SUFFIX}-gcc-${GCC_COMPILER_VERSION})
+find_program(CMAKE_CXX_COMPILER NAMES arm-linux-gnueabi${FLOAT_ABI_SUFFIX}-g++-${GCC_COMPILER_VERSION})
 set(ARM_LINUX_SYSROOT /usr/arm-linux-gnueabi${FLOAT_ABI_SUFFIX} CACHE PATH "ARM cross compilation system root")
 
 set(CMAKE_CXX_FLAGS           ""                    CACHE STRING "c++ flags")