doc: environment: NET_LWIP dhcp sets ipaddrN, netmaskN and gatewayipN

Document environment variables set by the dhcp command when the network
stack is lwIP.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/doc/usage/environment.rst b/doc/usage/environment.rst
index 7bd9ffc..30fc167 100644
--- a/doc/usage/environment.rst
+++ b/doc/usage/environment.rst
@@ -499,12 +499,12 @@
 -------------------------------
 
 The following environment variables may be used and automatically
-updated by the network boot commands ("bootp" and "rarpboot"),
+updated by the network boot commands ("bootp", "dhcp" and "rarpboot"),
 depending the information provided by your boot server:
 
-=========  ===================================================
+========== ===================================================================
 Variable   Notes
-=========  ===================================================
+========== ===================================================================
 bootfile   see above
 dnsip      IP address of your Domain Name Server
 dnsip2     IP address of your secondary Domain Name Server
@@ -514,7 +514,10 @@
 netmask    Subnet Mask
 rootpath   Pathname of the root filesystem on the NFS server
 serverip   see above
-=========  ===================================================
+ipaddrN    IP address for interface N (>0) (NET_LWIP dhcp only)
+netmaskN   Subnet mask for interface N (>0) (NET_LWIP dhcp only)
+gatewayipN IP address of the Gateway for interface N (>0) (NET_LWIP dhcp only)
+========== ===================================================================
 
 
 Special environment variables