kdat: allow to override build time

in order to allow for reproducible builds.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.
Signed-off-by: Bernhard M. Wiedemann <bwiedemann@suse.de>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/Makefile.crtools

 ccflags-y		+= -iquote criu/$(ARCH)
 ccflags-y		+= $(COMPEL_UAPI_INCLUDES)
 CFLAGS_REMOVE_clone-noasan.o += $(CFLAGS-ASAN)
-CFLAGS_kerndat.o	+= -DKDAT_MAGIC_2=${shell date +%s} -DKDAT_RUNDIR=\"$(RUNDIR)\"
+CFLAGS_kerndat.o	+= -DKDAT_MAGIC_2=${shell echo $${SOURCE_DATE_EPOCH:-$$(date +%s)}} -DKDAT_RUNDIR=\"$(RUNDIR)\"
 ldflags-y		+= -r
 
 obj-y			+= action-scripts.o