make: Added a placeholder makefile with some transition info.

Makefile

@@ -0,0 +1,17 @@
+all:
+	@echo "Makefile build system is depricated."
+	@echo ""
+	@echo "Please use Meson build system instead."
+	@echo ""
+	@echo "For most basic build, which builds only Black Magic Debug Application (BMDA), run the following commands:"
+	@echo "> meson setup build"
+	@echo "> meson compile -C build"
+	@echo ""
+	@echo "You can find example firmware configuration files in the \`cross-file\` subdirectory."
+	@echo "For example, to build a firmware for the native hardware run the following commands:"
+	@echo "> meson setup build-native --cross-file cross-file/native.ini --werror"
+	@echo "> meson compile -C build-native"
+	@echo ""
+	@echo "For further instructions please refer to the README.md in the root directory of this repository."
+	@echo ""
+	@exit 1