docs: Note which ports have default or optional network.PPP support.

Also add the default values of these macros to the respective
`mpconfigport.h` files, to improve discoverability.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

docs/library/network.PPP.rst

@@ -5,7 +5,12 @@ class PPP -- create network connections over serial PPP
 =======================================================
 
 This class allows you to create a network connection over a serial port using
-the PPP protocol.  It is only available on selected ports and boards.
+the PPP protocol.
+
+.. note:: Currently only the esp32 port has PPP support enabled in the default
+          firmware build. PPP support can be enabled in custom builds of the
+          stm32 and rp2 ports by enabling networking support and setting
+          ``MICROPY_PY_NETWORK_PPP_LWIP`` to 1.
 
 Example usage::
 

ports/rp2/mpconfigport.h

@@ -212,6 +212,10 @@
 #ifndef MICROPY_PY_WEBREPL
 #define MICROPY_PY_WEBREPL              (1)
 #endif
+
+#ifndef MICROPY_PY_NETWORK_PPP_LWIP
+#define MICROPY_PY_NETWORK_PPP_LWIP     (0)
+#endif
 #endif
 
 #if MICROPY_PY_NETWORK_CYW43

ports/stm32/mpconfigport.h

@@ -146,9 +146,6 @@
 #define MICROPY_HW_SOFTSPI_MAX_BAUDRATE (HAL_RCC_GetSysClockFreq() / 48)
 #define MICROPY_PY_WEBSOCKET        (MICROPY_PY_LWIP)
 #define MICROPY_PY_WEBREPL          (MICROPY_PY_LWIP)
-#ifndef MICROPY_PY_SOCKET
-#define MICROPY_PY_SOCKET           (1)
-#endif
 #ifndef MICROPY_PY_NETWORK
 #define MICROPY_PY_NETWORK          (1)
 #endif
@@ -156,6 +153,17 @@
 #define MICROPY_PY_ONEWIRE          (1)
 #endif
 
+// optional network features
+#if MICROPY_PY_NETWORK
+#ifndef MICROPY_PY_SOCKET
+#define MICROPY_PY_SOCKET           (1)
+#endif
+
+#ifndef MICROPY_PY_NETWORK_PPP_LWIP
+#define MICROPY_PY_NETWORK_PPP_LWIP     (0)
+#endif
+#endif
+
 // fatfs configuration used in ffconf.h
 #define MICROPY_FATFS_ENABLE_LFN       (2)
 #define MICROPY_FATFS_LFN_CODE_PAGE    437 /* 1=SFN/ANSI 437=LFN/U.S.(OEM) */