switchroot: Ensure /run/ostree-booted is created even without initramfs

switchroot: Ensure /run/ostree-booted is created even without initramfs

See https://mail.gnome.org/archives/ostree-list/2018-March/msg00012.html

If ostree-prepare-root is run as pid 1 (i.e we're not using an initramfs), then
anything we write outside the target sysroot (such as `/run/ostree-booted`) will
be lost.

Since `ostree-remount.service` runs fairly early in boot, and is triggered via
`ConditionKernelCommandLine=ostree`, we can just touch the file there in
addition.

Closes: #1508
Approved by: akiernan