Fix rp2 safe mode via reset press

The picodvi PR #7880 switched the saved word to the watchdog
register since it reworked the RAM layout. This works for
reset_into_safe_mode because the watchdog scratch registers are
preserved by soft resets. They *aren't* preserved for pressing the
reset button. So it broken manual safe mode. Switch back to using
RAM to store the saved word but use the pico-sdks "uninitialized"
designation instead of a fixed location.

Also fixes USB host feather status neopixel by setting the power
pin.

Author: tannewt

ports/raspberrypi/boards/adafruit_feather_rp2040_usb_host/mpconfigboard.h

@@ -2,6 +2,7 @@
 #define MICROPY_HW_MCU_NAME "rp2040"
 
 #define MICROPY_HW_NEOPIXEL (&pin_GPIO21)
+#define CIRCUITPY_STATUS_LED_POWER (&pin_GPIO20)
 
 #define DEFAULT_I2C_BUS_SCL (&pin_GPIO3)
 #define DEFAULT_I2C_BUS_SDA (&pin_GPIO2)

ports/raspberrypi/supervisor/port.c

@@ -247,14 +247,15 @@ uint32_t *port_heap_get_top(void) {
     return port_stack_get_top();
 }
 
+uint32_t __uninitialized_ram(saved_word);
 void port_set_saved_word(uint32_t value) {
-    // Store in a watchdog scratch register instead of RAM. 4-7 are used by the
-    // sdk. 0 is used by alarm. 1-3 are free.
-    watchdog_hw->scratch[1] = value;
+    // Store in RAM because the watchdog scratch registers don't survive
+    // resetting by pulling the RUN pin low.
+    saved_word = value;
 }
 
 uint32_t port_get_saved_word(void) {
-    return watchdog_hw->scratch[1];
+    return saved_word;
 }
 
 static volatile bool ticks_enabled;