kbuild: print only total number of section mismatces found

We have too many section mismatches detected at the moment.
So silence modpost and prevent the option from being
set in a typical allyesconfig build.

Tell the user how to see all the deteils in the summary
message from modpost.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 0d8a5a4..d090d72 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -81,7 +81,7 @@
 
 config DEBUG_SECTION_MISMATCH
 	bool "Enable full Section mismatch analysis"
-	default n
+	depends on UNDEFINED
 	help
 	  The section mismatch analysis checks if there are illegal
 	  references from one section to another section.