In working with platform libraries external from a "base" repo for which one is building, there have been issues with needing/not needing a trailing forward slash in the HDL_PLATFORM_PATH. The culprit is GNU make's 'notdir' function which strips a string containing a directory path of everything behind the last "/" - and if "/" is the last thing in the string, it unfortunately returns an empty string (see https://www.gnu.org/software/make/manual/html_node/File-Name-Functions.html). This 'notdir' function occurs multiple times in OpenCPI's Makefiles.
In my opinion, any environment variable that ever has a chance of getting passed through a "notdir" function in Makefiles should be preprocessed to have any trailing forward slashes stripped. That or we should avoid 'notdir' altogether, which I'm not sure if that is possible.