set OpenSBI dependencies right

[axel-h]

Follow-up from #117 concerning file(GLOB_RECURSE deps)

From @kent-mcleod: It looks like it's missing the other parts of it's implementation, actually specifying the path to the opensbi sources and then adding the sources to the depends line of add_custom_command. If you wanted to add a commit to this PR adding ${BBL_PATH} to the end of that statement, and also adding the deps arg to the ${CMAKE_BINARY_DIR}/bbl/bbl custom command [...]. Alternatively you could ignore it, or remove it if you think it's better to assume that the opensbi implementation isn't going to be edited by anyone between builds.