Support of COAP_PKI_KEY_PEM requires reading files which may not be supported
in libcoap running in a non Posix environment.
Remove RPK support when using PKI key type COAP_PKI_KEY_PEM. Update
coap-server and coap-client to read the optional .pem files into memory and
use PKI key type COAP_PKI_KEY_PEM_BUF instead.
Jon Shallow
25fbb26c60