certs: unify duplicated cmd_extract_certs and improve the log

certs: unify duplicated cmd_extract_certs and improve the log

cmd_extract_certs is defined twice. Unify them.

The current log shows the input file $(2), which might be empty.
You cannot know what is being created from the log, "EXTRACT_CERTS".

Change the log to show the output file with better alignment.

[Before]

  EXTRACT_CERTS   certs/signing_key.pem
  CC      certs/system_keyring.o
  EXTRACT_CERTS
  AS      certs/system_certificates.o
  CC      certs/common.o
  CC      certs/blacklist.o
  EXTRACT_CERTS
  AS      certs/revocation_certificates.o

[After]

  CERT    certs/signing_key.x509
  CC      certs/system_keyring.o
  CERT    certs/x509_certificate_list
  AS      certs/system_certificates.o
  CC      certs/common.o
  CC      certs/blacklist.o
  CERT    certs/x509_revocation_list
  AS      certs/revocation_certificates.o

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>