init-ifupdown: make sure the exit value of the script is 0

Rework the conditional expression so the overall return value of the
script is not failure, because in that case the 'ifup' command fails
with:

run-parts: /etc/network/if-up.d/ifup exited with code 1

Signed-off-by: Javier Viguera <javier.viguera@digi.com>

meta-digi-dey/recipes-core/init-ifupdown/init-ifupdown-1.0/ifup

@@ -30,4 +30,6 @@ eth1: | *:sms)
 	macaddr="$(sed -ne 's,^.*ethaddr3=\([^[:blank:]]\+\)[:blank:]*.*,\1,g;T;p' /proc/cmdline)"
 esac
 
-[ -n "${macaddr}" ] && ifconfig ${IFACE} hw ether ${macaddr} up
+if [ -n "${macaddr}" ]; then
+	ifconfig ${IFACE} hw ether ${macaddr} up
+fi