base-files: address sed in-place without SELinux awareness

base-files: address sed in-place without SELinux awareness

sed(1) in busybox does not support this functionality:
https://git.savannah.gnu.org/cgit/sed.git/tree/sed/execute.c#n598

This causes /etc/group to become mislabeled when a package requests
that a uid/gid be added on OpenWrt with SELinux

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
[move restorecon inside lock]
Signed-off-by: Dominick Grift <dominick.grift@defensec.nl>
(cherry picked from commit 5109bd164c8f2273329483f990188fb36cf3ad68)