kconfig: cleanup building information

Mute the following messages:
gcc -I/usr/include/ncursesw -DCURSES_LOC="<ncursesw/curses.h>" -DLOCALE
-Ibuild/host  -MM *.c > build/host/.depend 2>/dev/null || :
rm build/host/zconf.tab.c
mk: don't touch autoconf.h when exists

autoconf.h will be generated by mconf. There is no need to touch it.
Touch it every time will make the dependency checking failure.