linux-next-20210302: build problems with certs

I'm just guessing here, but I think that some of the certs build
infrastructure doesn't handle O=subdir, which is what I use all
of the time.

$ mkdir build64
$ make ARCH=x86_64 O=build64 all

  EXTRACT_CERTS   ../
At main.c:154:
- SSL error:0909006C:PEM routines:get_name:no start line: crypto/pem/pem_lib.c:745
extract-cert: ../: Is a directory
make[2]: [../certs/Makefile:119: certs/x509_revocation_list] Error 1 (ignored)


-- 
~Randy
Reported-by: Randy Dunlap <rdunlap@infradead.org>