Silence shellcheck warnings (#255)

* Made more specific shellcheck disables
* Fixed variable quoting (SC2086,SC2064)
* Use `$*` expansion instead of `$@` when not using arrays (SC2124)
* Use cleaner `$()` syntax instead of back quotes (SC2006)
* Improved comparator (SC2166)
* Minor improvements in coding style

Tested clean output using: `find -name "*.sh" | xargs -n1 shellcheck -x`.

export-noobs/00-release/files/partition_setup.sh

@@ -3,6 +3,7 @@
 
 set -ex
 
+# shellcheck disable=SC2154
 if [ -z "$part1" ] || [ -z "$part2" ]; then
   printf "Error: missing environment variable part1 or part2\n" 1>&2
   exit 1
@@ -17,7 +18,8 @@ sed /tmp/1/cmdline.txt -i -e "s|root=[^ ]*|root=${part2}|"
 sed /tmp/2/etc/fstab -i -e "s|^[^#].* / |${part2}  / |"
 sed /tmp/2/etc/fstab -i -e "s|^[^#].* /boot |${part1}  /boot |"
 
-if [ -z $restore ]; then
+# shellcheck disable=SC2154
+if [ -z "$restore" ]; then
   if [ -f /mnt/ssh ]; then
     cp /mnt/ssh /tmp/1/
   fi