py/mpconfig: Introduce reusable MP_HTOBE32(), etc. macros.

Macros to convert big-endian values to host byte order and vice-versa.
These were defined in adhoc way for some ports (e.g. esp8266), allow
reuse, provide default implementations, while allow ports to override.

ports/esp8266/posix_helpers.c

@@ -55,14 +55,13 @@ void *realloc(void *ptr, size_t size) {
     return p;
 }
 
-#define PLATFORM_HTONL(_n) ((uint32_t)( (((_n) & 0xff) << 24) | (((_n) & 0xff00) << 8) | (((_n) >> 8)  & 0xff00) | (((_n) >> 24) & 0xff) ))
 #undef htonl
 #undef ntohl
 uint32_t ntohl(uint32_t netlong) {
-    return PLATFORM_HTONL(netlong);
+    return MP_BE32TOH(netlong);
 }
 uint32_t htonl(uint32_t netlong) {
-    return PLATFORM_HTONL(netlong);
+    return MP_HTOBE32(netlong);
 }
 
 time_t time(time_t *t) {

py/mpconfig.h

@@ -1293,4 +1293,24 @@ typedef double mp_float_t;
 #define MP_UNLIKELY(x) __builtin_expect((x), 0)
 #endif
 
+#ifndef MP_HTOBE16
+#if MP_ENDIANNESS_LITTLE
+# define MP_HTOBE16(x) ((uint16_t)( (((x) & 0xff) << 8) | (((x) >> 8) & 0xff) ))
+# define MP_BE16TOH(x) MP_HTOBE16(x)
+#else
+# define MP_HTOBE16(x) (x)
+# define MP_BE16TOH(x) (x)
+#endif
+#endif
+
+#ifndef MP_HTOBE32
+#if MP_ENDIANNESS_LITTLE
+# define MP_HTOBE32(x) ((uint32_t)( (((x) & 0xff) << 24) | (((x) & 0xff00) << 8) | (((x) >> 8)  & 0xff00) | (((x) >> 24) & 0xff) ))
+# define MP_BE32TOH(x) MP_HTOBE32(x)
+#else
+# define MP_HTOBE32(x) (x)
+# define MP_BE32TOH(x) (x)
+#endif
+#endif
+
 #endif // MICROPY_INCLUDED_PY_MPCONFIG_H