Persistent memory check value verification, defaulting when fails. (#662)

* Make default constructor for touch calibration

* Add persistent memory check value and access abstraction.

* Add persistent data_t default constructor with reasonable defaults.

* serial_format_t default constructor.

* Tidy up backlight timeout type.

* Add persistent data struct version/checking.

* Make range_t functions constexpr.

* Move ui_config and functions into class.

* Add backlight_config_t struct, separate enable and time settings.

firmware/application/touch.hpp

@@ -137,6 +137,15 @@ struct Calibration {
 	{
 	}
 
+	constexpr Calibration() :
+		Calibration(
+			/* Values derived from one PortaPack H1 unit. */
+			{ { { 256, 731 }, { 880, 432 }, { 568, 146 } } },
+			{ { {  32,  48 }, { 208, 168 }, { 120, 288 } } }
+		)
+	{
+	}
+
 	ui::Point translate(const DigitizerPoint& p) const;
 
 private:
@@ -149,8 +158,6 @@ private:
 	int32_t f;
 };
 
-const Calibration default_calibration();
-
 template<size_t N>
 class Filter {
 public: