ostree-prepare-root: enabler for simpler kernel arg

ostree-prepare-root: enabler for simpler kernel arg

With the current approach, when ostree-prepare-root is used
on the kernel command line as init=, it always assumes that
the next value in the argument list is a path to the sysroot.
The code for falling back to a default path (if none is provided),
would only work if init= is the last arg in the argument list.
We can not rely on that and have to explicitly provide the
path to the sysroot. Which defeats the purpose of a default
path selection code.

To keep command line neater assume that sysroot is on / when
using ostree-prepare-root as init. This probably is what most
people want anyways. Also _ostree_kernel_args* API assumes
that args are space separated list. Which is problematic for:
"init=${ostree}/usr/lib/ostree/ostree-prepare-root /" as it
gets split in two.

Closes: #1401
Approved by: cgwalters