nrf: add SoftDevice support for the Circuit Playground Bluefruit

This also fixes a bug: the Bluefruit doesn't have a low frequency
crystal. Somehow non-SoftDevice code still worked. However, the
SoftDevice won't initialize when this flag is set incorrectly.

src/machine/board_circuitplay_bluefruit.go

@@ -2,7 +2,7 @@
 
 package machine
 
-const HasLowFrequencyCrystal = true
+const HasLowFrequencyCrystal = false
 
 // GPIO Pins
 const (

targets/circuitplay-bluefruit.json

@@ -1,6 +1,6 @@
 {
     "inherits": ["nrf52840"],
-    "build-tags": ["circuitplay_bluefruit","nrf52840_reset_uf2"],
+    "build-tags": ["circuitplay_bluefruit","nrf52840_reset_uf2", "softdevice", "s140v6"],
     "flash-1200-bps-reset": "true",
     "flash-method": "msd",
     "msd-volume-name": "CPLAYBTBOOT",

targets/circuitplay-bluefruit.ld

@@ -7,4 +7,7 @@ MEMORY
 
 _stack_size = 2K;
 
+/* This value is needed by the Nordic SoftDevice. */
+__app_ram_base = ORIGIN(RAM);
+
 INCLUDE "targets/arm.ld"