scripts/Makefile.build: do not reference EXTRA_CFLAGS as CFLAGS replacement

Usage of these flags has been deprecated for nearly 4 years by:

    commit f77bf014


    Author: Sam Ravnborg <sam@neptun.(none)>
    Date:   Mon Oct 15 22:25:06 2007 +0200

        kbuild: introduce ccflags-y, asflags-y and ldflags-y

Moreover, these flags (at least EXTRA_CFLAGS) have been documented for
command line use. By default, gmake(1) do not override command line
setting, so this is likely to result in build failure or unexpected
behavior.

Do not advertise for its usage.

Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>