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>

copy-firmware.sh

@@ -9,6 +9,7 @@ prune=no
 # shellcheck disable=SC2209
 compress=cat
 compext=
+destdir=
 
 while test $# -gt 0; do
     case $1 in
@@ -45,7 +46,7 @@ while test $# -gt 0; do
             ;;
 
         *)
-            if test "x$destdir" != "x"; then
+            if test -n "$destdir"; then
                 echo "ERROR: unknown command-line options: $*"
                 exit 1
             fi
@@ -56,7 +57,7 @@ while test $# -gt 0; do
     esac
 done
 
-if [ -z "$destdir" ]; then
+if test -z "$destdir"; then
 	echo "ERROR: destination directory was not specified"
 	exit 1
 fi