copy-firmware.sh: reset and consistently handle destdir
Currently we don't reset/override the destdir variable, so we end up inheriting
whatever the caller's environment has for it. While it may work, it's not
particularly consistent (be that within the script or other tools) nor is it
obvious.
While in here, ensure we handle the variable with test -z/-n instead of varying
other constructs.
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>