How to convert to and from a single PKCS12 certificate containing a cert, the key and the CA certs...
Converting a private key, a certificate and a CA chain to a single PKCS12 certificate bundle:
openssl pkcs12 -export -inkey cert-key.pem -in cert.crt -certfile ca_chain.pem -out cert.p12
Converting a PKCS12 certificate into separate files:
openssl pkcs12 -in cert.p12 -out cert-key.pem -nocerts -nodes
openssl pkcs12 -in cert.p12 -out cert.pem -clcerts -nokeys