We need to generate a "response file" to pass to the C preprocessor
when we preprocess the linker script, because of path length limitations
on Windows. We set the response file and bind the path to a global
property. The MBED_TARGET being built queries this global property when
it sets the linker script.

We must set this global property before the targets subdirectory is
added to the project. This is required because the MBED_TARGET depends
on the response file. If the path to the response file is not defined
when the target requests it the config definitions will not be passed
to CPP.