buildman -a should use config fragments

It dawned on me recently why buildman -a ~CMDLINE works differently than echo '# CONFIG_CMDLINE is not set' >> configs/sandbox_defconfig and that is because the former does a cycle of using the base defconfig, then modifying the .config whereas the latter modifies the config outright. If buildman instead made a tempfile to put the changes in and did "make defconfig /temp/changes.config" it would invoke merge_config.sh and get the same resulting .config as if the echo example had been done.