MdeModulePkg: Update IP4 stack to support point-to-point link with 31-bit mask.

V2 update: Directly use NetIp4IsUnicast() to check station address in IP driver.

This patch is to follow RFC3021 which allows to use 31-bit mask
in point-to-point link.
If a 31-bit subnet mask is assigned to a point-to-point link, it
leaves the <Host-number> with only 1 bit.  Consequently, only two
possible addresses may result:
  {<Network-number>, 0} and {<Network-number>, -1}
These addresses have historically been associated with network and
broadcast addresses (see Section 2.2).  In a point-to-point link with
a 31-bit subnet mask, the two addresses above MUST be interpreted as
host addresses.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>

MdeModulePkg/Include/Library/NetLib.h

@@ -414,8 +414,10 @@ NetGetIpClass (
 
   ASSERT if NetMask is zero.
   
-  If all bits of the host address of IP are 0 or 1, IP is also not a valid unicast address.
-
+  If all bits of the host address of IP are 0 or 1, IP is also not a valid unicast address,
+  except when the originator is one of the endpoints of a point-to-point link with a 31-bit
+  mask (RFC3021).
+  
   @param[in]  Ip                    The IP to check against.
   @param[in]  NetMask               The mask of the IP.