[nonet migration] adapt Makefile and github worklow, update build doc

This will permit to build on the github workflow both the 'net' and 'nonet'
variant

Makefile

@@ -6,8 +6,11 @@
 # append the Configuration variable to 'make' call with value to use in '/p:Configuration='
 # of msbuild command.
 #
+# append the Flavor variable to 'make' call with value to use in '/p:Flavor='
+# of msbuild command.
+#
 # Example:
-#    make Configuration=Release
+#    make Configuration=Release Flavor=NoNet
 #
 #
 # Some targets:
@@ -108,6 +111,11 @@ ifneq ($(Configuration),)
 else
   $(warning Configuration environment variable not set.)
 endif
+ifneq ($(Flavor),)
+  MSBUILD_PARAM += -p:Flavor="$(Flavor)"
+else
+  $(warning Flavor environment variable not set.)
+endif
 
 ifeq ($(detected_OS),Windows)
   define remove