Add cmake -DINIT_CONFIG= option that can point at any initial file.
"make clean" does not delete .config any more.
Note reconfiguration does NOT causes recompilation because -imacros
hides the generated .h from CMake's dependency scan. This is not a
regression, that problems exists since -imacros was introduced. At least
it's now possible to "make clean" and rebuild without losing the .config
file.
Fix for #3617
Signed-off-by: Marc Herbert <marc.herbert@intel.com>