Add build target to invoke the MNIST data download script

The `download-mnist` target  can be used as a dependency by any build
target that requires the MNIST handwritten digit dataset (for example,
test suites running integration tests for machine learning techniques).

No 'clean' option is provided since it is assumed that in general this
data should be downloaded once and re-used; the script will verify its
continued validity.

The download location can be specified to `make` by setting an explicit
`MNIST_DATA_DIR` variable, either in `Config.local.mak` or when invoking
`make` directly.  If this is not provided, the script will fall back to
using any predefined `MNIST_DATA_DIR` environment variable, and if that
is not available, to its default of `${HOME}/.cache/mnist`.

In general however this should not be necessary, since the defaults will
work fine for local testing, while CI setup can ensure `MNIST_DATA_DIR`
is defined if needed in that context (e.g. by explicitly setting it in
the docker image, or in the config for CI environment variables).

Author: joseph-wakeling-sociomantic

Build.mak

@@ -11,3 +11,7 @@ endif
 # use NaiveEngine (non-threaded) MXNet engine since the default (threaded)
 # version dead locks
 %test: export MXNET_ENGINE_TYPE=NaiveEngine
+
+.PHONY: download-mnist
+download-mnist: $C/script/download-mnist
+	$(call exec,sh $(if $V,,-x) $^,$(MNIST_DATA_DIR),$^)