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mkinitfs: add support for resume

This patch tries to add resume from hibernation (aka suspend to disk)
support to mkinitfs. The swap partition can be provided via the resume
option.

The default Alpine Linux kernel has no hibernation support compiled in,
so a custom kernel is needed in order to use this.

Some links on the topic:
* https://wiki.gentoo.org/wiki/Custom_Initramfs/Hibernation
* https://git.archlinux.org/mkinitcpio.git/tree/hooks/resume

--- a/initramfs-init.in

+++ b/initramfs-init.in

@@ -263,7 +263,7 @@

 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

-	overlaytmpfs rootfstype rootflags nbd"

+	overlaytmpfs rootfstype rootflags nbd resume"

 for opt; do

 	case "$opt" in

@@ -371,16 +371,25 @@

 	fi

 	ebegin "Mounting root"

+	nlplug-findfs $cryptopts -p /sbin/mdev $KOPT_root

+

+	if [ -n "$KOPT_resume" ]; then

+		echo "Resume from disk"

+		if [ -e /sys/power/resume ]; then

+			printf "%d:%d" $(stat -Lc "0x%t 0x%T" "$KOPT_resume") >/sys/power/resume

+		else

+			echo "resume: no hibernation support found"

+		fi

+	fi

+

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

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

 			$sysroot/media/root-rw

-		nlplug-findfs $cryptopts -p /sbin/mdev $KOPT_root \

-			&& mount -o ro $KOPT_root /media/root-ro

+		mount -o ro $KOPT_root /media/root-ro

 		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

-		nlplug-findfs $cryptopts -p /sbin/mdev $KOPT_root

 		mount ${KOPT_rootfstype:+-t} ${KOPT_rootfstype} \

 			-o ${KOPT_rootflags:-ro} \

 			$KOPT_root $sysroot