add hal or app level clock configuration (#62)

* add hal or app level clock configuration

* fix control flow and typo

* use hal.init(), thanks kuon for the improvement

src/core/microzig.zig

@@ -176,6 +176,15 @@ export fn microzig_main() noreturn {
     if (info.Fn.calling_convention == .Async)
         @compileError("TODO: Embedded event loop not supported yet. Please try again later.");
 
+    // A hal can export a default init function that runs before main for
+    // procedures like clock configuration. The user may override and customize
+    // this functionality by providing their own init function.
+    // function.
+    if (@hasDecl(app, "init"))
+        app.init()
+    else if (@hasDecl(hal, "init"))
+        hal.init();
+
     if (@typeInfo(return_type) == .ErrorUnion) {
         main() catch |err| {
             // TODO:

src/main.zig

@@ -160,10 +160,11 @@ pub fn addEmbeddedExecutable(
     exe.addPackage(chip_pkg);
     exe.addPackage(cpu_pkg);
 
-    if (options.hal_package_path) |hal_package_path|
     exe.addPackage(.{
         .name = "hal",
-            .source = hal_package_path,
+        .source = if (options.hal_package_path) |hal_package_path|
+            hal_package_path
+        else .{ .path = root_path ++ "core/empty.zig" },
         .dependencies = &.{pkgs.microzig},
     });