cpu: add documentation for DISABLE_BOARD_INIT/DISABLE_CPU_INIT

Author: benpicco

cpu/doc.txt

@@ -20,3 +20,27 @@
  * @ingroup     config
  * @brief       Compile time configurations for different kinds of CPU.
  */
+
+/**
+ * @brief   Skip calling `board_init()`
+ *
+ * Don't call `board_init()`, leave all pins in their default state.
+ *
+ * This is intended to be used with basic riotboot_slot which does not interact
+ * with any external hardware.
+ *
+ * @experimental    Only use this if you know what you are doing
+ */
+#define DISABLE_BOARD_INIT  0
+
+/**
+ * @brief   Skip calling `cpu_init()`
+ *
+ * Don't call `cpu_init()`, leave all CPU configuration in boot-up state.
+ *
+ * This is intended to be used with basic riotboot_slot which does not interact
+ * with any CPU peripherals.
+ *
+ * @experimental    Only use this if you know what you are doing
+ */
+#define DISABLE_CPU_INIT    0