homing_override: Add basic support for running custom g-code on G28

Allow users to override the behavior of G28 using a new
"homing_override" config section.  This may be used on printers that
require specific steps during the homing process.

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

config/example-extras.cfg

@@ -267,6 +267,19 @@
 #   default is to not scale the 'channel_x' parameters.
 
 
+# Homing override. One may use this mechanism to run a series of
+# g-code commands in place of a G28 found in the normal g-code input.
+# This may be useful on printers that require a specific procedure to
+# home the machine.
+#[homing_override]
+#gcode:
+#   A list of G-Code commands (one per line) to execute in place of
+#   all G28 commands found in the normal g-code input. If a G28 is
+#   contained in this list of commands then it will invoke the normal
+#   homing procedure for the printer. The commands listed here must
+#   home all axes. This parameter must be provided.
+
+
 # Replicape support - see the generic-replicape.cfg file for further
 # details.
 #[replicape]