File README.build.md of Package kiwi-builder-image

The following files are coming from _upstream_ https://build.opensuse.org/package/show/SUSE:SLFO:Products:SL-Micro:6.2/SL-Micro :

* SL-Micro.kiwi
* disk.sh
* config.sh
* editbootinstall_pine64.sh
* editbootinstall_rpi.sh

Those can be downloaded as:

```
curl -LO https://src.suse.de/products/SL-Micro/raw/branch/6.2/SL-Micro/SL-Micro.kiwi
```

The SL-Micro.kiwi file needs to be modified to append a few packages on the bootstrap stanza to be able to generate images with no SSL errors:

```
     <packages type="bootstrap">
         <package name="filesystem"/>
+        <package name="coreutils"/>
+        <package name="ca-certificates"/>
+        <package name="ca-certificates-mozilla"/>
     </packages>
```

The SL-Micro.kiwi.4096 file needs to be modified to modify the `target_blocksize="4096"` where appropiate.

All the other files are used verbatim.