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.
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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.