Update pi-util/BUILD.txt to better reflect reality

pi-util/BUILD.txt

@@ -1,29 +1,59 @@
 Building Pi FFmpeg
 ==================
 
-Configuration:
-=============
+Current only building on a Pi is supported.
+This builds ffmpeg the way I've tested it
 
-These instructions work for cross compiles from Ubuntu 16.04 & Ubuntu
-18.04. I would expect most other linux environments to work but I haven't
-tried them.
+Get all dependencies - the current package dependencies are good enough
 
-pi-util/conf_pi2.sh
+$ sudo apt-get build-dep ffmpeg
 
-contains suitable options to build the code for Pi2/3.  It expects to find
-git clones of
+Configure using the pi-util/conf_native.sh script
+-------------------------------------------------
 
-https://github.com/raspberrypi/tools
-https://github.com/raspberrypi/firmware
+This sets the normal release options and creates an ouutput dir to build into
+The directory name will depend on system and options but will be under out/
 
-in the parent of the FFmpeg directory.  I recommend using --depth 1 to avoid a
-lot of history you don't want.
+There are a few choices here
+ --mmal  build including the legacy mmal-based decoders and zero-copy code
+         this requires appropriate libraries which currently will exist for
+         armv7 but not arm64
+ --noshared
+         Build a static image rather than a shared library one.  Static is
+         easier for testing as there is no need to worry about library
+         paths being confused and therefore running the wrong code,  Shared
+         is what is needed, in most cases, when building for use by other
+         programs.
 
-If you have a copy of qasm.py in ../local/bin then the .qasm sources will be
-rebuilt.  Otherwise the prebuilt .c & .h files will be used.
-Likewise ../local/bin/vasmvidcore_std will enable VPU code rebuild
+So for a static build
+---------------------
 
-pi-util/conf_p1.sh should configure for Pi1.  Beware that as of this time
-H265 QPU acceleration is broken on Pi1 and so it is disabled.
+$ pi-util/conf_native.sh --noshared
+
+$ make -j8 -C out/<wherever the script said it was building to>
+
+You can now run ffmpeg directly from where it was built
+
+For a shared build
+------------------
+
+$ pi-util/conf_native.sh
+
+You will normally want an install target if shared. Note that the script has
+set this up to be generated in out/<builddir>/install, you don't have to worry
+about overwriting your system libs.
+
+$ make -j8 -C out/<builddir> install
+
+You can now set LD_LIBRARY_PATH appropriately and run ffmpeg from where it was
+built or install the image on the system - you have to be careful to get rid
+of all other ffmpeg libs or confusion may result.  There is a little script
+that wipes all other versions - obviously use with care!
+
+$ sudo pi-util/clean_usr_libs.sh
+
+Then simply copying from the install to /usr works
+
+$ sudo cp -r out/<builddir>/install/* /usr