Update doc/gitian-building.md for new gitian

The patch to make it possible to configure the LXC IPs has been merged
upstream. This means that it is no longer needed to patch gitian.
Remove that workaround.

doc/gitian-building.md

@@ -227,7 +227,6 @@ In this section we will be setting up the Debian installation for Gitian buildin
 First we need to log in as `root` to set up dependencies and make sure that our
 user can use the sudo command. Type/paste the following in the terminal:
 
-
 ```bash
 apt-get install git ruby sudo apt-cacher-ng qemu-utils debootstrap lxc python-cheetah parted kpartx bridge-utils
 adduser debian sudo
@@ -249,15 +248,16 @@ echo '#!/bin/sh -e' > /etc/rc.local
 echo 'brctl addbr br0' >> /etc/rc.local
 echo 'ifconfig br0 10.0.3.2/24 up' >> /etc/rc.local
 echo 'exit 0' >> /etc/rc.local
-# make sure that USE_LXC is always set when logging in as debian
+# make sure that USE_LXC is always set when logging in as debian,
+# and configure LXC IP addresses
 echo 'export USE_LXC=1' >> /home/debian/.profile
+echo 'export GITIAN_HOST_IP=10.0.3.2' >> /home/debian/.profile
+echo 'export LXC_GUEST_IP=10.0.3.5' >> /home/debian/.profile
 reboot
 ```
 
-At the end the VM is rebooted to make sure that the changes take effect.
-
-**Note**: If you're following this guide on a physical system instead of a VirtualBox VM you could use `10.0.2.2` instead
-of `10.0.3.2` in the above `ifconfig` line. This avoids having to patch gitian-builder in next section.
+At the end the VM is rebooted to make sure that the changes take effect. The steps in this
+section need only to be performed once.
 
 Installing gitian
 ------------------
@@ -286,20 +286,6 @@ git clone https://github.com/devrandom/gitian-builder.git
 git clone https://github.com/bitcoin/bitcoin
 ```
 
-We need to change the guest IP range for the gitian builder because otherwise it will
-collide with VirtualBox its NAT IP range. Gitian does not have a way yet to configure
-this, so we need to patch the IPs using `sed`. This is not nice but it will
-have to do for now... (a [pull request
-(#52)](https://github.com/devrandom/gitian-builder/pull/52) to make this
-configurable without patching has been submitted):
-
-```bash
-sed -i 's/10.0.2.2/10.0.3.2/g' gitian-builder/target-bin/bootstrap-fixup
-sed -i 's/10.0.2.5/10.0.3.5/g' gitian-builder/etc/lxc.config.in
-```
-
-*note* After you update the gitian-builder repository, you may need to repeat these manual changes.
-
 Setting up gitian images
 -------------------------