modem: Use readlink -f instead of realpath for set-madison-config

readlink is provided by coreutils, while realpath is in a separate package.

BUG=chromium-os:22117
TEST=Adhoc; modem set-madison-config still detects symlinks

Change-Id: I561967379e7bcd41fbe97935683bc171ace598a6
Reviewed-on: https://gerrit.chromium.org/gerrit/10906
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Tested-by: ttuttle <ttuttle@chromium.org>
1 file changed