doc: use KCFLAGS instead of EXTRA_CFLAGS to pass flags from command line

You should use KCFLAGS to pass additional compiler flags from the
command line. Using EXTRA_CFLAGS is wrong.

EXTRA_CFLAGS is supposed to specify flags applied only to the current
Makefile (and now deprecated in favor of ccflags-y).

It is still used in arch/mips/kvm/Makefile (and possibly in external
modules too). Passing EXTRA_CFLAGS from the command line overwrites
it and breaks the build.

I also fixed drivers/gpu/drm/tilcdc/Makefile because commit 816175dd1f
("drivers/gpu/drm/tilcdc: Makefile, only -Werror when no -W* in
EXTRA_CFLAGS") was based on the same misunderstanding.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Acked-by: Federico Vaga <federico.vaga@vaga.pv.it>
Link: https://lore.kernel.org/r/20210221152524.197693-1-masahiroy@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/4.Coding.rst

@@ -242,7 +242,7 @@ and try to avoid "fixes" which make the warning go away without addressing
 its cause.
 
 Note that not all compiler warnings are enabled by default.  Build the
-kernel with "make EXTRA_CFLAGS=-W" to get the full set.
+kernel with "make KCFLAGS=-W" to get the full set.
 
 The kernel provides several configuration options which turn on debugging
 features; most of these are found in the "kernel hacking" submenu.  Several

Documentation/process/submit-checklist.rst

@@ -96,7 +96,7 @@ and elsewhere regarding submitting Linux kernel patches.
     injection might be appropriate.
 
 20) Newly-added code has been compiled with ``gcc -W`` (use
-    ``make EXTRA_CFLAGS=-W``).  This will generate lots of noise, but is good
+    ``make KCFLAGS=-W``).  This will generate lots of noise, but is good
     for finding bugs like "warning: comparison between signed and unsigned".
 
 21) Tested after it has been merged into the -mm patchset to make sure