kbuild: ignore make's built-in rules & variables

kbuild does explicitly specify what to do in all cases, and each
time make's built-in rules & variables has been used it has been a bug.
So to speed up things and to avoid the hard-to-debug error situations
ignore the built-in definitions.
If any part of the kernel uses the built-in definitions the build will
just stop there and it should be trivial to fix.
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

Makefile

@@ -267,10 +267,9 @@ MAKEFLAGS += --include-dir=$(srctree)
 # We need some generic definitions
 include  $(srctree)/scripts/Kbuild.include
 
-# For maximum performance (+ possibly random breakage, uncomment
-# the following)
-
-#MAKEFLAGS += -rR
+# Do not use make's built-in rules and variables
+# This increases performance and avoid hard-to-debug behaviour
+MAKEFLAGS += -rR
 
 # Make variables (CC, etc...)