Several name fixes in Mender artifact uploads.
Several name fixes in Mender artifact uploads.

* Use board names for sd images instead of machine name, because one
  machine may be used for several boards.

* Replace legacy hardcoded vexpress references with board name
  variable.

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@northern.tech>

mendersoftware/meta-mender-qemu