base-files: functions.sh: replace [^...] with [!...] in case

Strictly speaking, ash does not support it.

From https://wiki.ubuntu.com/DashAsBinSh#A.5B.5E.5D

Not to be confused by sed's and other program's regular expression
syntax. Uses of [^...] in case (parameter/word expansion in general) need
to be replaced with [!...].

Found with shellcheck: https://github.com/koalaman/shellcheck/wiki/SC2169

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[minor commit title/message adjustments]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

package/base-files/files/lib/functions.sh

@@ -108,7 +108,7 @@ config_unset() {
 # config_get <section> <option>
 config_get() {
 	case "$2${3:-$1}" in
-		*[^A-Za-z0-9_]*) : ;;
+		*[!A-Za-z0-9_]*) : ;;
 		*)
 			case "$3" in
 				"") eval echo "\"\${CONFIG_${1}_${2}:-\${4}}\"";;