Some targets define MBEDTLS_md5_C in targets.json in order to force the system to use
external mbedtls instead of lwip internal crypt, i.e., polarssl.
LWIP's internal md5 mechanism is tied to PPP for some reason. In a previosly merged commit
an attempt was made to steal md5 functions by faking that PPP was turned on. However
that solution was broken in case of really turning on PPP functionality.
This commit fixes the breakage and also corrects the logic in case a target decides to use
external md5 implementation from mbedtls or otherwise (i.e, wants to stick to the internal
implementation).
lwip now uses mbed client random library under common pal when available.
Ported lwip reference TCP initial sequence number handling to mbed-os
lwip stack. Handling is based on RFC 6528.