cfg80211: make certificate generation more robust

stable inclusion
from stable-5.10.46
commit 5493b0c2a784ffa4c80c6a6e2c933824bfaba9b1
bugzilla: 168323
CVE: NA

--------------------------------

commit b5642479 upstream.

If all net/wireless/certs/*.hex files are deleted, the build
will hang at this point since the 'cat' command will have no
arguments. Do "echo | cat - ..." so that even if the "..."
part is empty, the whole thing won't hang.

Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210618133832.c989056c3664.Ic3b77531d00b30b26dcd69c64e55ae2f60c3f31e@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>