Add Sleep Mode

By devolov (though wouldn't have been possible without BSBob, SBird, and Kurausukun)

Goal: Allow a sleep function to be used to save battery when playing on a flashcart.

Warning: The sleep function won't work on mGBA and other emulators. It has been tested to work fine on an Everdrive, EZ Flash Omega DE, and EZ Flash IV.

In include/gba/io_reg.h, define the keys that you'd like to have the game go to sleep and wake up:

#define KEY_AND_INTR    0x8000
#define DPAD_ANY        ((DPAD_RIGHT | DPAD_LEFT | DPAD_UP | DPAD_DOWN))
#define JOY_EXCL_DPAD   0x030F

+#define SLEEP_KEYS      ((L_BUTTON | R_BUTTON | SELECT_BUTTON))
+#define WAKE_KEYS       ((SELECT_BUTTON | START_BUTTON))
+
// interrupt flags
#define INTR_FLAG_VBLANK  (1 <<  0)
#define INTR_FLAG_HBLANK  (1 <<  1)

In src/main.c, add the following to ReadKeys:

static void ReadKeys(void)
{
     u16 keyInput = REG_KEYINPUT ^ KEYS_MASK;
+
+    if (keyInput == SLEEP_KEYS)
+    {
+        vu16 IeBak, DispCntBak;
+        DispCntBak = REG_DISPCNT;      // LCDC OFF
+        REG_DISPCNT = 1 << 7;          // DISP_LCDC_OFF = 1 << 7
+        REG_KEYCNT= KEY_AND_INTR | KEY_INTR_ENABLE | WAKE_KEYS;
+        REG_IME = 0;
+        IeBak = REG_IE;               // IE save
+        REG_IE = INTR_FLAG_KEYPAD;    // Enable Key interrupt
+        REG_IME = 1;
+        asm("swi 0X03");              // This is the Stop command
+        REG_IME = 0;
+        REG_IE = IeBak;               // IE return
+        REG_IME = 1;
+        REG_DISPCNT = DispCntBak;     // LCDC ON
+        VBlankIntrWait();
+        while (keyInput)              // Doesn't continue until the wake keys are let go
+            keyInput = REG_KEYINPUT ^ KEYS_MASK;
+        return;
+    }
+
     gMain.newKeysRaw = keyInput & ~gMain.heldKeysRaw;
     gMain.newKeysReleased = ~keyInput & gMain.heldKeysRaw;
     gMain.newKeys = gMain.newKeysRaw;
     gMain.newAndRepeatedKeys = gMain.newKeysRaw;

And that's it! What you're doing is calling the BIOS Stop function (SWI 03) to put the GBA in low-power mode.
You're also setting the wake keys as a hardware interrupt to get the GBA out of Stop mode afterwards.