Contribution description

place compile_commands.json by rule target

Testing procedure

make compile-commands should result in the current compile_commands.json placement:
RIOTBASE if the app if in a subdirectory of that or APPDIR if not

make <somewhere>/compile_commands.json should place it <somewhere>

as i often have separate app an riot but they share a common path its usually

make ../../compile_commands.json or something like that

or

<somewhere>$ make -D <whatever to build> ./compile_commands.json

Issues/PRs references

#19869