net: Fully disable LWIP_ASSERTs

I was doing some debugging that had me looking at the disassembly of
lpc_rx_queue() from within the debugger.  I was looking for the call to
pbuf_alloc() that we see in the following code snippet:
		p = pbuf_alloc(PBUF_RAW, (u16_t) EMAC_ETH_MAX_FLEN, PBUF_RAM);
		if (p == NULL) {
			LWIP_DEBUGF(UDP_LPC_EMAC | LWIP_DBG_TRACE,
				("lpc_rx_queue: could not allocate RX pbuf (free desc=%d)\n",
				lpc_enetif->rx_free_descs));
			return queued;
		}

		/* pbufs allocated from the RAM pool should be non-chained. */
		LWIP_ASSERT("lpc_rx_queue: pbuf is not contiguous (chained)",
			pbuf_clen(p) <= 1);

When I was looking through the disassembly for this code I noticed a
call to pbuf_clen() in the actual machine code.
=> 0x0000bab0 <+24>:	bl	0x44c0 <pbuf_clen>
   0x0000bab4 <+28>:	ldr	r3, [r4, #112]	; 0x70
   0x0000bab6 <+30>:	ldrh.w	r12, [r5, #10]
   0x0000baba <+34>:	add.w	r2, r3, #9
   0x0000babe <+38>:	add.w	r0, r12, #4294967295	; 0xffffffff

The only call to pbuf_clean made from this function is made from
within the LWIP_ASSERT.  When I looked more closely at how this macro
was defined, I saw that the mbed version of the stack had disabled the
LWIP_PLATFORM_ASSERT macro when LWIP_DEBUG was false which means that
no action will be taken if the assert is false but it still allows the
LWIP_ASSERT macro to potentially evaluate the assert expression.
Defining the LWIP_NOASSERT macro will fully disable the LWIP_ASSERT
macro.

I saw one of my TCP/IP samples shrink about 0.5K when I made this
change.

libraries/net/lwip/lwip/lwipopts.h

@@ -99,6 +99,7 @@
 #define MEMP_OVERFLOW_CHECK         1
 #define MEMP_SANITY_CHECK           1
 #else
+#define LWIP_NOASSERT               1
 #define LWIP_STATS                  0
 #endif