docker: Don't use eval trick on Makefile

The eval trick for defining DOCKER_SRC_COPY doesn't do anything
useful, as DOCKER_SRC_COPY is immediately expanded just after it
is defined, and CUR_TIME is already defined using ":=". Simply
define it using ":=" so it is evaluated only once.

The eval trick was also triggering an weird error on Travis builds:
  qemu/tests/docker/Makefile.include:34: *** unterminated variable reference.  Stop.

The issue is not easily reproducible (maybe it's a bug in some
versions of Make), but it is avoided if removing the eval trick.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>