support/scripts: allow /etc/shadow to be symlinked

support/scripts: allow /etc/shadow to be symlinked

This commit fixes a problem where it was not possible to replace
/etc/shadow with a symlink to a e.g. a user partition where the
shadow file is placed. This is required, e.g. for systems where the
rootfs is mounted read-only but users should still be able to be
added. Thus, if within an filesystem overlay setup a user tries
to replace /etc/shadow with a symlink to the real file on a user
partition a buildroot build stops with an error message because
sed is called on the symlink instead of following the symlink.
This commit fixes this shortcoming.

Signed-off-by: Jens Maus <mail@jens-maus.de>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>