stm32/rng: Use Yasmarang for rng_get() if MCU doesn't have HW RNG.

ports/stm32/rng.c

@@ -60,4 +60,30 @@ STATIC mp_obj_t pyb_rng_get(void) {
 }
 MP_DEFINE_CONST_FUN_OBJ_0(pyb_rng_get_obj, pyb_rng_get);
 
+#else // MICROPY_HW_ENABLE_RNG
+
+// For MCUs that don't have an RNG we still need to provide a rng_get() function,
+// eg for lwIP.  A pseudo-RNG is not really ideal but we go with it for now.  We
+// don't want to use urandom's pRNG because then the user won't see a reproducible
+// random stream.
+
+// Yasmarang random number generator by Ilya Levin
+// http://www.literatecode.com/yasmarang
+STATIC uint32_t pyb_rng_yasmarang(void) {
+    static uint32_t pad = 0xeda4baba, n = 69, d = 233;
+    static uint8_t dat = 0;
+
+    pad += dat + d * n;
+    pad = (pad << 3) + (pad >> 29);
+    n = pad | 2;
+    d ^= (pad << 31) + (pad >> 1);
+    dat ^= (char)pad ^ (d >> 8) ^ 1;
+
+    return pad ^ (d << 5) ^ (pad >> 18) ^ (dat << 1);
+}
+
+uint32_t rng_get(void) {
+    return pyb_rng_yasmarang();
+}
+
 #endif // MICROPY_HW_ENABLE_RNG