scripts/mender-qemu: allow image name to be passed in command line
scripts/mender-qemu: allow image name to be passed in command line

User can pass image name in command line. If not provided, fallback to
core-image-full-cmdline.

Signed-off-by: Maciej Borzecki <f609d9f713abcd5f8f0e51f08dc151638a7681e7@open-rnd.pl>

bboozzoo/meta-mender,bboozzoo/meta-mender,GregorioDiStefano/meta-mender,GregorioDiStefano/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender,GregorioDiStefano/meta-mender