Created (a new) PPP interface for PPP service. Removed lwip
dependencies to PPP (memory allocations etc.). Moved PPP
configuration options away from lwIP mbed_lib.json to new
PPP service. For backwards compatibility, using the old
options is also currently supported.
Save some ROM space by putting MBED_NORETURN attributes on error
functions and failed asserts.
mbed_error was documented as returning an error code. It never
actually could return, so documentation updated, but return type
kept.