x86: simplify "make ARCH=x86" and fix kconfig all.config

Simplify "make ARCH=x86" and fix kconfig so we again can set 64BIT in
all.config.

For a fix the diffstat is nice:
 6 files changed, 3 insertions(+), 36 deletions(-)

The patch reverts these commits:
 - 0f855aa6 ("kconfig: add helper to set
   config symbol from environment variable")
 - 2a113281 ("kconfig: use $K64BIT to
   set 64BIT with all*config targets")

Roman Zippel pointed out that kconfig supported string compares so
the additional complexity introduced by the above two patches were
not needed.

With this patch we have following behaviour:

  # make {allno,allyes,allmod,rand}config [ARCH=...]
  option \ host arch      | 32bit         | 64bit
  =====================================================
  ./.                     | 32bit         | 64bit
  ARCH=x86                | 32bit         | 32bit
  ARCH=i386               | 32bit         | 32bit
  ARCH=x86_64             | 64bit         | 64bit

The general rule are that ARCH= and native architecture takes
precedence over the configuration.

So make ARCH=i386 [whatever] will always build a 32-bit kernel
no matter what the configuration says.  The configuration will
be updated to 32-bit if it was configured to 64-bit and the
other way around.

This behaviour is consistent with previous behaviour so no
suprises here.

make ARCH=x86 will per default result in a 32-bit kernel but as
the only ARCH= value x86 allow the user to select between 32-bit
and 64-bit using menuconfig.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Andreas Herrmann <aherrman@arcor.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>