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

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>