coap_malloc_type implementation for lwip based on memp

those things need to reside in mem.h (instead of living implementation
dependently inside mem.c) because coap_malloc_type(MY_TYPE, ...) is
converted to memp_malloc(MEMP_MY_TYPE) and thus needs to be done in the
preprocessor. (the alternative would be to switch on the MY_TYPE enum,
which would be harder to maintain).

mem.h

@@ -13,12 +13,14 @@
 #include <stdlib.h>
 #endif
 
+#ifndef WITH_LWIP
 /**
  * Initializes libcoap's memory management. This function must be
  * called once before coap_malloc() can be used on constrained
  * devices.
  */
 void coap_memory_init(void);
+#endif /* WITH_LWIP */
 
 /**
  * Type specifiers for coap_malloc_type(). Memory objects can be typed
@@ -29,6 +31,8 @@ typedef enum {
   COAP_STRING, COAP_ATTRIBUTE_NAME, COAP_ATTRIBUTE_VALUE, COAP_NODE, COAP_CONTEXT, COAP_ENDPOINT
 } coap_memory_tag_t;
 
+#ifndef WITH_LWIP
+
 /**
  * Allocates a chunk of @p size bytes and returns a pointer to the
  * newly allocated memory. The @p type is used to select the
@@ -67,4 +71,26 @@ static inline void coap_free(void *object) {
   coap_free_type(COAP_STRING, object);
 }
 
+#endif /* not WITH_LWIP */
+
+#ifdef WITH_LWIP
+
+#include <lwip/memp.h>
+
+/* no initialization needed with lwip (or, more precisely: lwip must be
+ * completely initialized anyway by the time coap gets active)  */
+static inline void coap_memory_init(void) {}
+
+/* it would be nice to check that size equals the size given at the memp
+ * declaration, but i currently don't see a standard way to check that without
+ * sourcing the custom memp pools and becoming dependent of its syntax */
+#define coap_malloc_type(type, size) memp_malloc(MEMP_ ## type)
+#define coap_free_type(type, p) memp_free(MEMP_ ## type, p)
+
+/* those are just here to make uri.c happy where string allocation has not been made conditional */
+static inline void *coap_malloc(size_t size) { LWIP_ASSERT("coap_malloc must not be used in lwIP", 0); }
+static inline void coap_free(void *pointer) { LWIP_ASSERT("coap_free must not be used in lwIP", 0); }
+
+#endif /* WITH_LWIP */
+
 #endif /* _COAP_MEM_H_ */