kbuild: clarify "make C=" build option

Clarify the use of "make C=" in the top-level Makefile, and fix a
typo in the Documentation file.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/Documentation/sparse.txt b/Documentation/sparse.txt
index 5a311c3..f9c99c9 100644
--- a/Documentation/sparse.txt
+++ b/Documentation/sparse.txt
@@ -69,10 +69,10 @@
 be recompiled or not.  The latter is a fast way to check the whole tree if you
 have already built it.
 
-The optional make variable CF can be used to pass arguments to sparse.  The
-build system passes -Wbitwise to sparse automatically.  To perform endianness
-checks, you may define __CHECK_ENDIAN__:
+The optional make variable CHECKFLAGS can be used to pass arguments to sparse.
+The build system passes -Wbitwise to sparse automatically.  To perform
+endianness checks, you may define __CHECK_ENDIAN__:
 
-        make C=2 CF="-D__CHECK_ENDIAN__"
+        make C=2 CHECKFLAGS="-D__CHECK_ENDIAN__"
 
 These checks are disabled by default as they generate a host of warnings.