Revert "config.mk: Add MK_VIMAGE knob"

This commit broke "make makeman" checks in github CI due to a lack of
option description files. The split between VIMAGE and VIMAGE_SUPPORT
is not clearly justified and the code is broken because there is no
opt_vimage.h (it's in opt_global.h).

This reverts commit 22ca6db50f4e6bd75a141f57cf953d8de6531a06.