overlay.d/05core: support booting ISO completely from RAM

It has been presented to us a use case where the user wants to actually
"install" directly to the same device that was used to boot the live
ISO image (i.e. the ISO was written to a block device and then booted
from). The way the ISO boots today it mounts the ISO at /run/media/iso
and then mounts the CoreOS rootfs from a file under that mountpoint. In
this scenario /run/media/iso will be busy and can't be unmounted because
the root filesystem of the live running system depends on it.

We can adjust the strategy slightly to get a system that runs completely
from memory (similar to live PXE systems) by just copying the file out
of /run/media/iso and into memory before doing the rootfs mount, which
will allow /run/media/iso to be unmounted before the installation
commences.

This patchset adds a new coreos.liveiso.fromram option to allow
requesting this new behavior. We could consider making this new
behavior the default, but we'd need to consider the new RAM requirements.
For example, before this change on a 2G FCOS system the usage after
booting looks like:

```
[core@localhost ~]$ free -m
               total        used        free      shared  buff/cache   available
Mem:            1953         163        1286         125         503        1519
Swap:              0           0           0
```

With this change (and the coreos.liveiso.fromram kernel arguemnt
specified) the usage looks like:

```
[core@localhost ~]$ free -m
               total        used        free      shared  buff/cache   available
Mem:            1953         173         688         808        1091         824
Swap:              0           0           0
```

The usage would scale with the size of the rootfs. i.e. RHCOS is larger
so the memory requirements would be larger too.

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-live/coreos-liveiso-persist-osmet.service

@@ -1,10 +1,12 @@
 [Unit]
 Description=Persist Osmet Files (ISO)
-DefaultDependencies=false
 ConditionPathExists=/run/ostree-live
 ConditionKernelCommandLine=coreos.liveiso
 RequiresMountsFor=/run/media/iso
 Before=initrd-switch-root.target
+# DefaultDependencies=true so this unit gets stopped on switchroot to
+# allow for /run/media/iso to get unmounted.
+DefaultDependencies=true
 
 [Service]
 Type=oneshot

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-live/live-generator

@@ -98,6 +98,9 @@ Wants=systemd-udev-settle.service
 After=sysinit.target
 After=initrd-root-device.target
 Before=initrd-root-fs.target
+# This will make /run/media/iso get unmounted when no longer needed,
+# which will aid in the coreos.liveiso.fromram case.
+StopWhenUnneeded=true
 
 [Mount]
 What=/${isosrc}
@@ -106,16 +109,44 @@ Options=ro
 Type=iso9660
 EOF
 
+    # Determine what to mount in sysroot.mount based on if we were asked to run
+    # completely from RAM via coreos.liveiso.fromram karg.
+    if getargbool 0 coreos.liveiso.fromram; then
+        sysrootrequiresmountsfor=""
+        sysrootfsimg=/rootfs.img
+        # Add service to copy the rootfs.img from the ISO to memory
+        cat >"${UNIT_DIR}/coreos-liveiso-run-media-iso-cp-rootfsimg.service" <<EOF
+# Automatically generated by live-generator
+
+[Unit]
+RequiresMountsFor=/run/media/iso
+Before=sysroot.mount
+# DefaultDependencies=true so this unit gets stopped on switchroot to
+# allow for /run/media/iso to get unmounted.
+DefaultDependencies=true
+
+[Service]
+Type=oneshot
+ExecStart=cp -v /run/media/iso/images/pxeboot/rootfs.img /rootfs.img
+RemainAfterExit=yes
+EOF
+        add_requires coreos-liveiso-run-media-iso-cp-rootfsimg.service default.target
+    else
+        sysrootrequiresmountsfor="RequiresMountsFor=/run/media/iso"
+        sysrootfsimg=/run/media/iso/images/pxeboot/rootfs.img
+    fi
+
     cat >"${UNIT_DIR}/sysroot.mount" <<EOF
 # Automatically generated by live-generator
 
 [Unit]
 DefaultDependencies=false
 Before=initrd-root-fs.target
-RequiresMountsFor=/run/media/iso
+After=coreos-liveiso-run-media-iso-cp-rootfsimg.service
+${sysrootrequiresmountsfor}
 
 [Mount]
-What=/run/media/iso/images/pxeboot/rootfs.img
+What=${sysrootfsimg}
 Where=/sysroot
 Type=squashfs
 # Offset of the squashfs within the rootfs cpio.  Assumes newc format