checkpatch.pl: Add a U-Boot option

Add an option to indicate that U-Boot-specific checks should be enabled.
Add a function to house the code that will be added.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/.checkpatch.conf b/.checkpatch.conf
index 95f1963..ed0c215 100644
--- a/.checkpatch.conf
+++ b/.checkpatch.conf
@@ -28,3 +28,6 @@
 
 # A bit shorter of a description is OK with us.
 --min-conf-desc-length=2
+
+# Extra checks for U-Boot
+--u-boot