kconfig: use $K64BIT to set 64BIT with all*config targets

The variable K64BIT can now be used to select the
value of CONFIG_64BIT.

This is for example useful for powerpc to generate
allmodconfig for both bit sizes - like this:
make ARCH=powerpc K64BIT=y
make ARCH=powerpc K64BIT=n

To use this the Kconfig file must use "64BIT" as the
config value to select between 32 and 64 bit.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Roman Zippel <zippel@linux-m68k.org>

README

@@ -194,6 +194,8 @@ CONFIGURING the kernel:
    "make *config" checks for a file named "all{yes/mod/no/random}.config"
    for symbol values that are to be forced.  If this file is not found,
    it checks for a file named "all.config" to contain forced values.
+   Finally it checks the environment variable K64BIT and if found, sets
+   the config symbol "64BIT" to the value of the K64BIT variable.
    
 	NOTES on "make config":
 	- having unnecessary drivers will make the kernel bigger, and can