Hikey: Add flash and nvme targets

Adds two new targets, one that downloads the necessary nvme.img file and
another target that flash the device. We have also added a couple of
.PHONY directives to a few targets.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Victor Chong <victor.chong@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>