fix argument number for custom linker script (ARMmbed#169)

Author: JojoS62

tools/cmake/mbed_target_functions.cmake

@@ -134,8 +134,8 @@ function(mbed_set_post_build target)
             )
             endif()
         else()
-            message(STATUS "${target} uses custom linker script  ${ARGV2}")
-            mbed_set_custom_linker_script(${target} ${ARGV2})
+            message(STATUS "${target} uses custom linker script  ${ARGV1}")
+            mbed_set_custom_linker_script(${target} ${ARGV1})
         endif()
     endif()