Change the I2C timeout to a choice (#402)

* Change the I2C timeout to a choice to reduce the number of options in the config editor

* Ensure /config exists before trying to save to it

software/firmware/europi_config.py

@@ -139,7 +139,7 @@ def config_points(cls):
                 choices=[0, 1],
                 default=0,
                 danger=True,
-            ),            
+            ),
             configuration.choice(
                 name="DISPLAY_FREQUENCY",
                 choices=[
@@ -182,11 +182,25 @@ def config_points(cls):
                 ],
                 default=100000,
             ),
-            configuration.integer(
+            configuration.choice(
                 name="EXTERNAL_I2C_TIMEOUT",
-                minimum=0,
-                maximum=5000,
-                default=1000
+                choices=[
+                    # somewhat arbitrary subset of 0-5000
+                    # this should cover a good range of use-cases
+                    0,
+                    1,
+                    5,
+                    10,
+                    25,
+                    50,
+                    100,
+                    250,
+                    500,
+                    1000,
+                    2500,
+                    5000,
+                ],
+                default=1000,
             ),
 
             # I/O voltage settings

software/firmware/experimental/settings_menu.py

@@ -25,6 +25,7 @@
 from experimental.knobs import KnobBank
 from framebuf import FrameBuffer, MONO_HLSB
 from machine import Timer
+import os
 import time
 
 
@@ -740,6 +741,12 @@ def save(self, settings_file):
         data = {}
         for item in self.menu_items_by_name.values():
             data[item.config_point.name] = item.value_choice
+        try:
+            # ensure the /config directory exists
+            # save_to_file won't create it for us!
+            os.mkdir("config")
+        except OSError:
+            pass
         ConfigFile.save_to_file(settings_file, data)
         self.settings_dirty = False