udhcpc: tweak math shell style with the metric var

Some shells (like dash) are lame and omit the POSIX increment/decrement
feature (because it is listed as optional).  Tweak the shell script to
work in all POSIX variants.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

examples/udhcp/simple.script

@@ -29,7 +29,8 @@ case "$1" in
 			metric=0
 			for i in $router ; do
 				echo "Adding router $i"
-				route add default gw $i dev $interface metric $((metric++))
+				route add default gw $i dev $interface metric $metric
+				: $(( metric += 1 ))
 			done
 		fi