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support for tmpfs overlay over root

useful if wanting to optimize boot time, and do device updates
via read-only system image. package management is not too useful
in these situations.

--- a/features.d/base.modules

+++ b/features.d/base.modules

@@ -1,1 +1,2 @@

 kernel/drivers/block/loop.ko

+kernel/fs/overlayfs

--- a/initramfs-init.in

+++ b/initramfs-init.in

@@ -387,7 +387,8 @@

 myopts="alpine_dev autodetect autoraid chart cryptroot cryptdm debug_init

 	dma init_args keep_apk_new modules ovl_dev pkgs quiet root_size root

-	usbdelay ip alpine_repo apkovl alpine_start splash blacklist"

+	usbdelay ip alpine_repo apkovl alpine_start splash blacklist

+	overlaytmpfs"

 for opt; do

 	case "$opt" in

@@ -544,7 +545,16 @@

 		start_$i

 	done

 	ebegin "Mounting root"

-	retry_mount -o ro $KOPT_root $sysroot 2>/dev/null

+	if [ "$KOPT_overlaytmpfs" = "yes" ]; then

+		mkdir -p /media/root-ro /media/root-rw $sysroot/media/root-ro $sysroot/media/root-rw

+		retry_mount -o ro $KOPT_root /media/root-ro 2>/dev/null

+		mount -t tmpfs root-tmpfs /media/root-rw

+		mkdir -p /media/root-rw/work /media/root-rw/root

+		mount -t overlay -o lowerdir=/media/root-ro,upperdir=/media/root-rw/root,workdir=/media/root-rw/work overlayfs $sysroot

+	else

+		retry_mount -o ro $KOPT_root $sysroot 2>/dev/null

+	fi

+

 	eend $?

 	cat /proc/mounts | while read DEV DIR TYPE OPTS ; do

 		if [ "$DIR" != "/" -a "$DIR" != "$sysroot" -a -d "$DIR" ]; then