build system: Revert all the recent additions of MK[...] knobs that
allow conditionally disabling the building of certain user space
programs in the 'base' set.

There is not enough consensus that this is the right way and a few
people had strong objections, see source-changes-d@.

mk: Allow setting MKDHCPD=no to build base without the ISC DHCP server,
useful for embedded images that don't need to act as one.