Add MGBA debug logging w/ puts

MGBA debugging must be enabled using debug::open and disabled using
debug::close. This commit adds logging through debug::puts. TODO: add
logging using debug::printf and debug::write.

include/debug.hpp

@@ -0,0 +1,16 @@
+#include <cstdint>
+
+namespace debug {
+enum level {
+	fatal = 0,
+	error = 1,
+	warn = 2,
+	info = 3,
+	debug = 4,
+};
+
+void puts(const char* msg, level lvl = level::info );
+
+bool open();
+void close();
+};

src/debug.cpp

@@ -0,0 +1,49 @@
+#include "debug.hpp"
+
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+
+#include <tonc.h>
+
+namespace debug {
+/**
+ * \brief MGBA debug registers and constants
+ */
+namespace reg {
+/**
+ * \brief MGBA debug enable register.
+ *
+ * When 0xC0DE is written to the register, debug output is enabled.
+ * When 0 is written to the register, debug output is disabled.
+ * Will be set to 0x1DEA when successfully enabled.
+ */
+static volatile uint16_t* enable = reinterpret_cast<uint16_t*>(0x4FFF780);
+/**
+ * \brief MGBA debug flags register.
+ */
+static volatile uint16_t* flags = reinterpret_cast<uint16_t*>(0x4FFF700);
+/**
+ * \brief MGBA debug string register.
+ *
+ * Max length of 0x100 (256)
+ */
+static char* string = reinterpret_cast<char*>(0x4FFF600);
+constexpr size_t string_len = 0x100;
+}; // namespace reg
+
+
+void puts(const char* msg, level lvl) {
+	std::strncpy(reg::string, msg, reg::string_len);
+	*reg::flags = (lvl) | 0x100;
+}
+
+bool open() {
+	*reg::enable = 0xC0DE;
+	return *reg::enable ==0x1DEA;
+}
+
+void close() {
+	*reg::enable = 0;
+}
+}; // namespace debug