Update some README files; Plus minor simplification to a preceding ch…

…ange.

configs/b-l475e-iot01a/README.txt

@@ -609,3 +609,8 @@ Configuration sub-directories
         if multiple radios ACK?  At a minimum it could keep the driver
         unnecessarily busy.  There is some prototype code to do just this
         in the driver, but does not seem to work.
+
+      2017-08-24:  There is only a single buffer for reassemblying larger
+        packets.  This could be an important issue for the hub configuration
+        which really needs the capability concurrently reassemble multiple
+        incoming streams.

configs/clicker2-stm32/README.txt

@@ -686,9 +686,13 @@ Configurations
         some additional fixes for byte ordering in 16-bit and 64-bit
         compressed IPv6 addresses, then all tests are working as expected:
         TCP, UDP, Telnet.
-      2017-08-5:  It looks like I have lost one of my Clicker2-STM32 boards.
+      2017-08-05:  It looks like I have lost one of my Clicker2-STM32 boards.
         This means that I will not be able to do any regression testing as
         changes are made to the radio interfaces and 6LoWPAN :(
+      2017-08-24:  There is only a single buffer for reassemblying larger
+        packets.  This could be an important issue for the hub configuration
+        which really needs the capability concurrently reassemble multiple
+        incoming streams.
 
   nsh:
 

configs/same70-xplained/README.txt

@@ -1383,6 +1383,11 @@ Configuration sub-directories
 
         No significant functional testing has yet been performed.
 
+      2017-08-24:  There is only a single buffer for reassemblying larger
+        packets.  This could be an important issue for the hub configuration
+        which really needs the capability concurrently reassemble multiple
+        incoming streams.
+
   netnsh:
 
     Configures the NuttShell (nsh) located at examples/nsh.  There are two

configs/samv71-xult/README.txt

@@ -1956,6 +1956,11 @@ Configuration sub-directories
         The SPI signals look clean on the board and the MRF24J40 seems
         fully functional.
 
+      2017-08-24:  There is only a single buffer for reassemblying larger
+        packets.  This could be an important issue for the hub configuration
+        which really needs the capability concurrently reassemble multiple
+        incoming streams.
+
   mxtxplnd:
 
     Configures the NuttShell (nsh) located at examples/nsh.  There are five

include/nuttx/net/netconfig.h

@@ -70,33 +70,15 @@
 #  define MIN(a,b) ((a) < (b) ? (a) : (b))
 #endif
 
-/* Try to eliminate dependencies on other header files.  This should not harm
- * portability because these are well-known constants anyway.
+/* Eliminate dependencies on other header files.  This should not harm
+ * portability because these are well-known constants.
  */
 
-#ifdef IPv4_HDRLEN
-#  define __IPv4_HDRLEN IPv4_HDRLEN
-#else
-#  define __IPv4_HDRLEN 20
-#endif
-
-#ifdef IPv6_HDRLEN
-#  define __IPv6_HDRLEN IPv6_HDRLEN
-#else
-#  define __IPv6_HDRLEN 40
-#endif
-
-#ifdef UDP_HDRLEN
-#  define __UDP_HDRLEN  UDP_HDRLEN
-#else
-#  define __UDP_HDRLEN  8
-#endif
-
-#ifdef TCP_HDRLEN
-#  define __TCP_HDRLEN  TCP_HDRLEN
-#else
-#  define __TCP_HDRLEN  20  /* REVISIT: Not really a constant */
-#endif
+#define __IPv4_HDRLEN 20  /* Must match IPv4_HDRLEN in include/nuttx/net/ip.h */
+#define __IPv6_HDRLEN 40  /* Must match IPv4_HDRLEN in include/nuttx/net/ip.h */
+#define __UDP_HDRLEN  8   /* Must match UDP_HDRLEN in include/nuttx/net/dup.h */
+#define __TCP_HDRLEN  20  /* Must match TCP_HDRLEN in include/nuttx/net/tcp.h */
+                          /* REVISIT: Not really a constant */
 
 /* Layer 2 Configuration Options ********************************************/