certs: avoid irrelevant compilation

[glimchb]

for make:

diff --git a/docker-compose.yml b/docker-compose.yml
index aaf88d6..ee45042 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -110,7 +110,8 @@ services:
         echo "DNS.3 = web" >> sztpd1/sbi/end-entity/openssl.cnf
         echo "DNS.4 = redirecter" >> sztpd1/sbi/end-entity/openssl.cnf
         sed -i 's/my-serial-number/third-serial-number/g' client/end-entity/openssl.cnf
-        make pki SHELL=/bin/ash
+        make -C sztpd1/sbi pki SHELL=/bin/ash
+        make -C client     pki SHELL=/bin/ash
         echo SERVER SBI certificates
         cat sztpd1/sbi/end-entity/my_cert.pem sztpd1/sbi/intermediate2/my_cert.pem > /tmp/cert_chain.pem
         openssl crl2pkcs7 -nocrl -certfile /tmp/cert_chain.pem -outform DER -out /tmp/cert_chain.cms
(END)

and for tar:

-        tar -zxvf sztpd-simulator-0.0.11.tgz -C /tmp
+        tar -zxvf sztpd-simulator-0.0.11.tgz -C /tmp sztpd-simulator/pki/sztpd1/sbi sztpd-simulator/pki/client