In bb_config.h, replace AUTOCONF_INCLUDED guard with the more reasonable and

In bb_config.h, replace AUTOCONF_INCLUDED guard with the more reasonable and
obvious (and less side-effect prone in strange build environments) BB_CONFIG_H.

Yeah, I know Erik ripped it out of our copy of menuconfig (which is a good
thing), but that doesn't fix people whose headers have it inherited from
linux-kernel headers or old versions of uclibc, and Erik's fix could easily
get forgotten and reverted the next time we update menuconfig anyway...