service/pm: Implement SetMaintenanceBoot()

This quite literally functions as a basic setter. No other error
checking or anything (since there's nothing to really check against).

With this, it completes the pm:bm interface in terms of functionality.

src/core/hle/service/pm/pm.cpp

@@ -13,7 +13,7 @@ public:
     explicit BootMode() : ServiceFramework{"pm:bm"} {
         static const FunctionInfo functions[] = {
             {0, &BootMode::GetBootMode, "GetBootMode"},
-            {1, nullptr, "SetMaintenanceBoot"},
+            {1, &BootMode::SetMaintenanceBoot, "SetMaintenanceBoot"},
         };
         RegisterHandlers(functions);
     }
@@ -27,6 +27,15 @@ private:
         rb.PushEnum(boot_mode);
     }
 
+    void SetMaintenanceBoot(Kernel::HLERequestContext& ctx) {
+        LOG_DEBUG(Service_PM, "called");
+
+        boot_mode = SystemBootMode::Maintenance;
+
+        IPC::ResponseBuilder rb{ctx, 2};
+        rb.Push(RESULT_SUCCESS);
+    }
+
     SystemBootMode boot_mode = SystemBootMode::Normal;
 };