Update the README according to the new build infrastructure.

no-tn-check

Change-Id: I7b6df87cb63f3a4c1fa5bf0eacc0efacc0d7e0fd

Author: lambourg

README.md

@@ -9,32 +9,41 @@ This repository depends on both gcc and gnat repositories to generate a full
 runtime. However it can also use the sources from an installed compiler and
 rely on those to just generate the BSP part of the runtime.
 
-## generation of BSPs
+You need GNAT Pro 21.0w 20200327 or GNAT Community 2020 minimum to use
+this repository to generate runtimes.
+
+## generation of runtimes
 
 ```
-./build_rts.py --output=temp --bsps-only <board1> <board2> ...
+./build_rts.py --output=temp --build <board1> <board2> ...
 ```
 
 The list of supported boards is listed in build_rts.py within build_configs.
 
-## building and installing a runtime
+The above call with generate the runtimes for <board1> <board2> in 'temp'
+and will build them, assuming the proper compiler is in the PATH.
 
-Once a BSP is generated, make sure you have setup a GNAT compiler for the
-board's target, and call gprbuild/gprinstall
+To install the runtime in the compiler's default location, you will thus
+need to specify
 
 ```
-gprbuild -P temp/BSPs/<rts_project>.gpr -j0 -f
-gprinstall -P temp/BSPs/<rts_project>.gpr -p -f
+./build_rts.py --output <gnat_prefix>/<target>/lib/gnat` ...
 ```
 
-where -P specified the project file, -j0 specifies to build using all CPUs
-available on the host, -f forces a full project build.
+So for example --output /opt/gnat/arm-eabi/lib/gnat
 
-## rts with debug information
+## rebuild of a runtime
 
-To build a runtime with debug information, you can set the scenario variable
-BUILD to Debug:
+To build a runtime with non default options, use the project file present in
+the runtime folder: runtime_build.gpr for zfp runtimes and ravenscar_build.gpr
+for ravenscar runtimes.
+
+To build with debug options, use -XBUILD=Debug ; to build with assertions use
+-XBUILD=Assert.
+
+So for example to rebuild the ravenscar-sfp-stm32f4 runtime with debug
+information, assuming GNAT is installed in ~/install/gnat, run the following:
 
 ```
-gprbuild -P temp/BSPs/<rts_project>.gpr -j0 -XBUILD=Debug -f
-```
+gprbuild -P ~/install/gnat/arm-eabi/lib/gnat/ravenscar-sfp-stm32f4/ravenscar-build.gpr -XBUILD=Debug
+```