feat(iso): Add bash-only CIDR calculator function (#21)

setup.sh

@@ -36,6 +36,17 @@ trap '[ "$?" -ne 77 ] || exit 77' ERR
 
 declare zpool_drive efi_drive boot_drive part_schema
 
+function calculate_prefix_from_netmask () {
+    # https://stackoverflow.com/a/50419919
+    c='0'
+    x='0'"$(printf -- '%o' ${1//./ })"
+    while [ "${x}" -gt '0' ]; do
+        # Modulo then bitwise-shift x and store as new x
+        let c+="$(( x % 2 ))" 'x>>=1'
+    done
+    printf -- '%s' '/'"${c}";
+}
+
 function setup_env_vars () {
     printf -- '%s\n' \
         'We will go over a series of questions to create an answer file with' \