pbuf_type PPP is using for LCP, PAP, CHAP, EAP, IPCP and IP6CP packets.
Memory allocated must be single buffered for PPP to works, it requires pbuf
that are not going to be chained when allocated. This requires setting
PBUF_POOL_BUFSIZE to at least 512 bytes, which is quite huge for small systems.
Setting PPP_USE_PBUF_RAM to 1 makes PPP use memory from heap where continuous
buffers are required, allowing you to use a smaller PBUF_POOL_BUFSIZE.