stepper: Support configuring the stepper pulse length from Kconfig

Allow the stepper pulse length to be configured at compile time from
the Kconfig menu system.

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

src/Kconfig

@@ -2,6 +2,13 @@
 
 mainmenu "Klipper Firmware Configuration"
 
+config LOW_LEVEL_OPTIONS
+    bool "Enable extra low-level configuration options"
+    default n
+    help
+        Enable low-level configuration options that (if modified) may
+        result in a build that does not function correctly.
+
 choice
     prompt "Micro-controller Architecture"
     config MACH_AVR
@@ -32,6 +39,29 @@ source "src/linux/Kconfig"
 source "src/simulator/Kconfig"
 
 
+# Step timing customization
+config CUSTOM_STEP_DELAY
+    bool "Specify a custom step pulse duration"
+    depends on LOW_LEVEL_OPTIONS
+config STEP_DELAY
+    int
+    default 2
+config STEP_DELAY
+    int "Step pulse duration (in microseconds)"
+    depends on CUSTOM_STEP_DELAY
+    help
+        Specify the duration of the stepper step pulse time. This
+        setting applies to all stepper drivers controlled by the
+        micro-controller. If this value is set to zero then the code
+        will "step" and "unstep" in the same C function.
+
+        The default is zero for 8-bit AVR based micro-controllers, as
+        it takes a little over 2us to step and unstep with this
+        setting.
+
+        The default for all other micro-controllers is 2us.
+
+        CUSTOMIZING THIS VALUE DOES NOT IMPROVE PERFORMANCE!
 
 # The HAVE_GPIO_x options allow boards to disable support for some
 # commands if the hardware does not support the feature.
@@ -54,13 +84,6 @@ config HAVE_GPIO_BITBANGING
     bool
     default n
 
-config NO_UNSTEP_DELAY
-    # Slow micro-controllers do not require a delay before returning a
-    # stepper step pin to its default level.  A board can enable this
-    # option to optimize the stepper_event() handler in this case.
-    bool
-    default n
-
 config INLINE_STEPPER_HACK
     # Enables gcc to inline stepper_event() into the main timer irq handler
     bool