Add boot logo bypass feature

Pressing the RIGHT button while the boot logo is scrolling down will
abort the sequence, to start running the actual sketch faster.

README.md

@@ -54,6 +54,8 @@ A user settable *unit name* of up to 6 characters can be saved in system EEPROM
 
 Once the logo display sequence completes, the sketch continues.
 
+For developers who wish to quickly begin testing, or impatient users who want to go strait to playing their game, the boot logo sequence can be bypassed by holding the *RIGHT* button while powering up, and then releasing it. Alternatively, the *RIGHT* button can be pressed while the logo is scrolling down.
+
 ### "Flashlight" mode
 
 If the *UP* button is pressed and held when the Arduboy is powered on, it enters *flashlight* mode. This turns the RGB LED fully on, and all the pixels of the screen are lit, resulting in a bright white light suitable as a small flashlight. (For an incorrect RGB LED, only the screen will light). To exit *flashlight* mode, press the *DOWN* button to continue with the sketch.
@@ -193,6 +195,11 @@ void Arduboy2Base::begin()
   audio.begin();
 
   bootLogo();
+
+  // wait for all buttons to be released
+  do {
+    delay(50);
+  } while (buttonsState());
 }
 ```
 

src/Arduboy2.cpp

@@ -45,6 +45,11 @@ void Arduboy2Base::begin()
   audio.begin();
 
   bootLogo();
+
+  // wait for all buttons to be released
+  do {
+    delay(50);
+  } while (buttonsState());
 }
 
 void Arduboy2Base::flashlight()
@@ -93,6 +98,11 @@ void Arduboy2Base::bootLogo()
   digitalWrite(RED_LED, RGB_ON);
 
   for (int8_t y = -18; y <= 24; y++) {
+    if (pressed(RIGHT_BUTTON)) {
+      digitalWriteRGB(RGB_OFF, RGB_OFF, RGB_OFF); // all LEDs off
+      return;
+    }
+
     if (y == -4) {
       digitalWriteRGB(RGB_OFF, RGB_ON, RGB_OFF); // green LED on
     }

src/Arduboy2.h

@@ -236,6 +236,11 @@ class Arduboy2Base : public Arduboy2Core
    * The Arduboy logo scrolls down from the top of the screen to the center
    * while the RGB LEDs light in sequence.
    *
+   * If the RIGHT button is pressed while the logo is scrolling down,
+   * the boot logo sequence will be aborted. This can be useful for
+   * developers who wish to quickly start testing, or anyone else who is
+   * impatient and wants to go straight to the actual sketch.
+   *
    * This function calls `bootLogoExtra()` after the logo stops scrolling down,
    * which derived classes can implement to add additional information to the
    * logo screen. The `Arduboy2` class uses this to display the unit name.