overlay: fix compilation with glibc

glibc fails with:
warn_unused_result [-Wunused-result]
   system("cp -a /tmp/root/upper/* / 2>/dev/null");

Regression intoroduced with 11efbf3b90f031ce634fc38cad07a4aef2985777
Reported here:
https://www.mail-archive.com/lede-dev@lists.infradead.org/msg10502.html

Signed-off-by: Roman Yeryomin <roman@advem.lv>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>