Apply snap-preseed optimizations after seeding snaps

The snap-preseed command can do a number of things during the build
that are currently performed at first boot (apparmor profiles, systemd
unit generation, etc).  This patch adds a call to reset the seeding and
apply these optimizations when adding a seeded snap.  As a prerequisite
to calling snap-preseed we need to make /dev/mem available as well as
mounts from the host to perform this work, so those are also added here.

(cherry picked from commit 1ca11c9795)

live-build/auto/build

@@ -112,7 +112,22 @@ preinstall_snaps() {
 	for snap in "$@"; do
 		SNAP_NO_VALIDATE_SEED=1 snap_preseed chroot "${snap}"
 	done
+
+	if [ ! -c /dev/mem ]; then
+		mknod -m 660 /dev/mem c 1 1
+		chown root:kmem /dev/mem
+	fi
+
+	mount --rbind /dev chroot/dev
+	mount --rbind /sys chroot/sys
+	mount --bind /proc chroot/proc
+
 	snap_validate_seed chroot
+
+	umount --recursive chroot/proc
+	umount --recursive chroot/sys
+	umount --recursive chroot/dev
+
 	lb chroot_resolv remove
 }
 

live-build/functions

@@ -96,9 +96,15 @@ mount_image() {
 setup_mountpoint() {
     local mountpoint="$1"
 
+    if [ ! -c /dev/mem ]; then
+        mknod -m 660 /dev/mem c 1 1
+        chown root:kmem /dev/mem
+    fi
+
     mount --rbind /dev "$mountpoint/dev"
     mount proc-live -t proc "$mountpoint/proc"
     mount sysfs-live -t sysfs "$mountpoint/sys"
+    mount securityfs -t securityfs "$mountpoint/sys/kernel/security"
     mount -t tmpfs none "$mountpoint/tmp"
     mount -t tmpfs none "$mountpoint/var/lib/apt"
     mount -t tmpfs none "$mountpoint/var/cache/apt"
@@ -682,6 +688,8 @@ snap_validate_seed() {
 
     if [ -e "${CHROOT_ROOT}/var/lib/snapd/seed/seed.yaml" ]; then
         snap debug validate-seed "${CHROOT_ROOT}/var/lib/snapd/seed/seed.yaml"
+        /usr/lib/snapd/snap-preseed --reset $(realpath "${CHROOT_ROOT}")
+        /usr/lib/snapd/snap-preseed $(realpath "${CHROOT_ROOT}")
     fi
 }