Doing another 'make' after a 'make clean' was broken until now. Fix it
by not deleting build/config.h during 'make clean' (only in 'make distclean').
Also, change the default behaviour of 'make' from asking the user to
run 'make config' (or similar) to actually _run_ 'make config' without
asking questions. It's always possible to explicitly invoke
'make menuconfig' or 'make xconfig' and so on, of course.
Finally, make _all_ targets (allyesconfig, randconfig, and so on)
generate a build/config.h file, as we always #include it.
Signed-off-by: Uwe Hermann <email@example.com>
Acked-by: Jordan Crouse <firstname.lastname@example.org>