Experimental overlayfs support

If aufs not available/broken we could try to use overlayfs.
NB: changes below doesn't work without modified make-initrd-propagator!

features.in/stage2/stage1/modules.d/10-stage2

@@ -34,6 +34,7 @@ atl2.ko
 atp870u.ko
 atp.ko
 aufs.ko
+overlay.ko
 axnet_cs.ko
 b44.ko
 bnx2.ko

features.in/stage2/stage1/scripts.d/03-test-kernel

@@ -24,7 +24,8 @@ configured()
 ls $CONFIGS >&/dev/null || fatal "no stage1 kernel config found in /boot"
 
 # test for installer-required filesystems support
-for opt in SQUASHFS AUFS_FS $GLOBAL_STAGE1_KCONFIG; do
+for opt in SQUASHFS AUFS_FS OVERLAY_FS $GLOBAL_STAGE1_KCONFIG; do
+	[ "$opt" = AUFS_FS ] && [ $(configured "OVERLAY_FS=[my]") -a $(configured "AUFS_FS=n") ] && continue
 	configured "$opt=[my]" || fatal "stage1 kernel must have $opt support"
 done
 

sub.in/stage1/modules

@@ -4,6 +4,7 @@ kernel/drivers/rtc
 af_packet.ko
 arc4.ko
 aufs.ko
+overlay.ko
 cdrom.ko
 evdev.ko
 ext4.ko