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>

scripts/kconfig/conf.c

@@ -591,6 +591,7 @@ int main(int ac, char **av)
 			conf_read_simple(name, S_DEF_USER);
 		else if (!stat("all.config", &tmpstat))
 			conf_read_simple("all.config", S_DEF_USER);
+		conf_set_env_sym("K64BIT", "64BIT", S_DEF_USER);
 		break;
 	default:
 		break;