Improve generated Makefile from `mbed export -i make_gcc_arm`

I have a bit of a strange workflow wherein I use `mbed compile` to actually generate the code for my application but I need to generate a compilation database (a `compile_commands.json` file) to be used with `clang` tools.

To do this, I use `mbed export -i make_gcc_arm` to generate a Makefile, and then I use https://github.com/rizsotto/Bear to generate the `compile_commands.json` file. Bear is simply a `make` wrapper so the command used is just `bear -- make`.

I typically have to `make clean` before running the bear command which wipes out the entire `./BUILD` directory. This means subsequent `mbed compile` commands have to be run from scratch. Is it possible to generate the Makefile so that the output directory is something like `./BUILD/make`?