gpiocmds: Add Kconfig option to allow boards to disable ADC commands

Some boards may not support the ADC hardware.  Update the build so
that those commands do not need to be compiled if they are not
available.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/Kconfig

@@ -13,6 +13,12 @@ endchoice
 source "src/avr/Kconfig"
 source "src/simulator/Kconfig"
 
+# The HAVE_GPIO_x options allow boards to disable support for some
+# commands if the hardware does not support the feature.
+config HAVE_GPIO_ADC
+    bool
+    default n
+
 config INLINE_STEPPER_HACK
     # Enables gcc to inline stepper_event() into the main timer irq handler
     bool