zram-swap: bail out early if the kernel doesn't support swap

Since KERNEL_SWAP is only enabled by default for !SMALL_FLASH targets, we need
to check if the current kernel supports swap before trying to configure
zram-swap, as opkg can't check for kernel dependencies.

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
(cherry picked from commit 565dfeb128)

package/system/zram-swap/files/zram.init

@@ -150,6 +150,11 @@ zram_compact()
 
 start()
 {
+	[ -e /proc/swaps ] || {
+		logger -s -t zram_start -p daemon.crit "kernel doesn't support swap"
+		return 1
+	}
+
 	if [ $( grep -cs zram /proc/swaps ) -ne 0 ]; then
 		logger -s -t zram_start -p daemon.notice "[OK] zram swap is already mounted"
 		return 1