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 gener...