Prior to this fix, CMAKE_NMC_DEFAULT_CONFIGS would inherit "all" from
the union of CMAKE_NMC_DEFAULT_BUILD_FILE_CONFIG and
CMAKE_NMC_CROSS_CONFIGS. This is inconsistent with the behavior of the
"all" target signifying CMAKE_NMC_CROSS_CONFIGS. Update "all" in
CMAKE_NMC_DEFAULT_CONFIGS to inherit only from CMAKE_NMC_CROSS_CONFIGS.
