Update openwrt instructions with the toolchain setup

packages/openwrt/README.md

@@ -1,3 +1,20 @@
+## Prerequisites
+
+This instructions explain how to build an OpenWRT .ipk package for n2n.
+
+Before going on, it is required to have a working cross-compiling build
+environment for the OpenWRT version installed into your device. This usually
+comes down to the following steps:
+
+1. Download and extract the SDK toolchain for your device. The toolchain
+   must match the *exact* OpenWRT version installed in your device. Toolchain
+   for official OpenWRT images can be downloaded from https://downloads.openwrt.org
+
+2. Build the toolchain: run `make menuconfig`, save the configuration, then
+   run `make` to build the cross compiling tools
+
+3. Download the feeds with `./scripts/feeds update -a`
+
 ## Compilation
 
 From the OpenWRT build directory:
@@ -9,6 +26,10 @@ make menuconfig # select Network -> VPN -> n2n-edge and n2n-supernode
 make package/n2n/compile V=s
 ```
 
+If everything went fine, two ipk will be generated, one for the n2n-edge
+and the other for n2n-supernode. They can be found with `find . -name "n2n*.ipk"`,
+copied to the target device, and installed with `opkg install`.
+
 ## Configuration
 
 The edge node can be started with `/etc/init.d/edge start`.