templates: add 'system-flake' template

templates/system-flake/pkgs/hello-world/Makefile

@@ -0,0 +1,6 @@
+CFLAGS += -O3 -s
+
+all: hello-world
+
+%: %.c
+	$(CC) $(CFLAGS) -o $@ $<

templates/system-flake/pkgs/hello-world/default.nix

@@ -0,0 +1,14 @@
+{stdenv, ...}:
+stdenv.mkDerivation {
+  name = "hello-world";
+  version = "1.0.0";
+
+  src = ./.;
+
+  installPhase = ''
+    mkdir -p $out/bin
+    cp hello-world $out/bin
+  '';
+
+  meta.mainProgram = "hello-world";
+}

templates/system-flake/pkgs/hello-world/hello-world.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+
+int main()
+{
+	printf("Hello, world!\n");
+	return 0;
+}