botutil: botutil_public.c shall not include bootutil_private.h

Objective for existence botutil_public.c is to provide common code
for MCUboot itself and for an chain-loaded application. Therefor
it shouldn't depend on MCUboot exclusive functions or so.

This path moves missing part of bootutil_private.h to bootutil_private.c

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>