Update 'Build Qt5.12.10 for RPI3 X11 and EGLFS'

Build-Qt5.12.10-for-RPI3-X11-and-EGLFS.md

@@ -2,7 +2,7 @@
 1. Get the source code from:
 https://download.qt.io/official_releases/qt/5.12/5.12.10/
 
-2. Uncomment developement source in 
+2. Uncomment developement source (deb-src) in 
 ```
 sudo nano /etc/apt/sources.list
 ```
@@ -29,9 +29,15 @@ sudo apt-get install gstreamer1.0-omx libgstreamer1.0-dev libgstreamer-plugins-b
 sudo mkdir /usr/local/qt5pi
 sudo chown pi:pi /usr/local/qt5pi
 ```
-7. 
-
-build configuration:
+7. [on Host] Create a sysroot. Using rsync we can properly keep things synchronized in the future as well. Replace raspberrypi.local with the address of the Pi.
+```
+mkdir sysroot sysroot/usr sysroot/opt
+rsync -avz pi@raspberrypi.local:/lib sysroot
+rsync -avz pi@raspberrypi.local:/usr/include sysroot/usr
+rsync -avz pi@raspberrypi.local:/usr/lib sysroot/usr
+rsync -avz pi@raspberrypi.local:/opt/vc sysroot/opt
+```
+8. [on Host]build configuration:
 ```
 ../configure -release -no-eglfs -qt-xcb -xcb -nomake tests -nomake examples -skip wayland -skip qtscript -skip qtwebengine \
 -device linux-rasp-pi3-g++ \