ci/bare-metal: make BM_BOOTCONFIG optional
In the RPI farm, all the runners set the exact same BM_BOOTCONFIG; let's make the variable optional so that we can just move these to the default file and drop the variable from all the runners.
In the RPI farm, all the runners set the exact same BM_BOOTCONFIG; let's make the variable optional so that we can just move these to the default file and drop the variable from all the runners.