Improve homescreen backlighting to depend on external light

Instead of relying on what power remains in the battery, rely instead on the external lighting to adapt the brightness of the screen.
Note that this might need a bit more fine-tuning.

apps/home/main.py

@@ -71,11 +71,11 @@ def draw_wifi(back_colour, rssi, connected, connecting, win_wifi):
 def backlight_adjust():
 	if ugfx.backlight() == 0:
 		ugfx.power_mode(ugfx.POWER_ON)
-	l = pyb.ADC(16).read()
-	if (l > 90):
-		ugfx.backlight(100)
-	elif (l > 20):
-		ugfx.backlight(70)
+	l = onboard.get_light()
+
+	lum_percent = int(l * 100 / 4096)
+	if (lum_percent > 30):
+		ugfx.backlight(lum_percent)
 	else:
 		ugfx.backlight(30)
 
@@ -298,11 +298,9 @@ def home_main():
 				inactivity_limit = 120
 			else:
 				inactivity_limit = 30
-			if battery_percent > 120:  #if charger plugged in
-				if ugfx.backlight() == 0:
-					ugfx.power_mode(ugfx.POWER_ON)
-				ugfx.backlight(100)
-			elif inactivity > inactivity_limit:
+
+			# If charger is not plugged in and we're inactive, turn screen off
+			if battery_percent < 120 and inactivity > inactivity_limit:
 				low_power()
 			else:
 				backlight_adjust()