Update cross compile instructions

Closes: #184

libdbus-sys/cross_compile.md

@@ -1,9 +1,9 @@
 Cross compiling dbus
 ====================
 
-Disclaimer: I'm writing this because people ask me how to do it, not because I'm a subject matter expert. I hope that someone more knowledgable will submit a PR which fills in some of the blanks here as I haven't fully succeeded myself yet.
+The examples below all assume you're trying to compile for Raspberry Pi 2 or 3 running Raspbian. Adjust target triples accordingly if your target is something else.
 
-(The examples below all assume you're trying to compile for Raspberry Pi 2 or 3 running Raspbian. Adjust target triples accordingly if your target is something else.) 
+Before you start: there is an alternative guide at [issue 184](https://github.com/diwic/dbus-rs/issues/184#issuecomment-520228758), which also contains a [powershell script](https://github.com/diwic/dbus-rs/issues/184#issuecomment-520791888) that set things up for you. Feel free to use either this guide or the alternative one as you see fit.
 
 A cross linker
 --------------
@@ -32,21 +32,22 @@ This one's easy, just run rustup:
 Target dbus libraries
 ---------------------
 
-**Installing the library**
+**Installing the libraries**
 
 Now to the more challenging part. Since we link to a C library `libdbus-1.so`, we also need the target version of that library. However, `libdbus-1.so` in itself links to a systemd library (at least it does so here) which in turn links to other libraries etc, so we need target versions of those libraries too.
 
-Getting an entire rootfs/image is probably the easiest option. The rootfs needs to have `libdbus-1-dev` installed.
+Getting an entire rootfs/image is probably the easiest option. The rootfs needs to have `libdbus-1-dev` installed. I e:
 
-[//]: # (Ugly hack: sudo mount -o loop,offset=50331648 2019-04-08-raspbian-stretch-lite.img /tmp/mnt ) 
-[//]: # (cd /tmp/mnt/usr/lib/arm-linux-gnueabihf && ln -s ../../../lib/arm-linux-gnueabihf/libdbus-1.so.3 libdbus-1.so ) 
+ * Boot your target (i e, a raspberry), install `libdbus-1-dev` on it, turn it off and put the SD card into your computer's SD card reader. Mount the partition.
+ * If the above is not an option, you could download an image, mount it at the right offset, like this (ugly hack!) `sudo mount -o loop,offset=50331648 2019-04-08-raspbian-stretch-lite.img /tmp/mnt` and then, to manually make the symlink that `libdbus-1-dev` does for you: `cd /tmp/mnt/usr/lib/arm-linux-gnueabihf && ln -s ../../../lib/arm-linux-gnueabihf/libdbus-1.so.3 libdbus-1.so`.
+ * Or you can use the alternative guide's approach to download, extract and post-process the relevant `.deb` files manually. This might be a preferrable option if an entire image/rootfs would be too large.
 
-**Finding the library**
+**Finding the libraries**
 
 When not cross compiling, finding the right library is done by a `build.rs` script which calls `pkg-config`. This will not work when cross compiling because it will point to the `libdbus-1.so` on the host, not the `libdbus-1.so` of the target.
 Maybe it is possible to teach `pkg-config` how to return the target library instead, but I have not tried this approach. Instead we can override build script altogether and provide the same info manually. This is possible because `libdbus-sys` has a `links = dbus` line.
 
-For the example below we assume that we have mounted a Raspbian rootfs on `/tmp/mnt`, and that the cross linker came with some basic libraries (libc, libpthread etc) that are installed on `/usr/arm-linux-gnueabihf/lib`.
+For the example below we assume that we have mounted a Raspbian rootfs on `/tmp/mnt`, and that the cross linker came with some basic libraries (libc, libpthread etc) that are installed on `/usr/arm-linux-gnueabihf/lib`. Unfortunately, we can't use the basic libraries that are present on the image, because they might contain absolute paths.
 
 And so we add the following to [.cargo/config](https://doc.rust-lang.org/cargo/reference/config.html):