- Replace all users of `$(RIOTBASE)/build` with the already present
`$(BUILD_DIR)` variable
- Replace all users of `$(BUILD_DIR)/pkg` with the already present
`$(PKGDIRBASE)` variable
- Create a `CACHEDIR.TAG` file in the `$(BUILD_DIR)`
Makefiles don't do comments, so these were forwarded into the variable.
*Most* users would expand the arguments to a shell where it'd be
ignored, but not all of them.
Contributes-To: https://github.com/RIOT-OS/RIOT/pull/18489
(This is also where the one version that is added here was removed).
