lib/sysroot: Support /usr/lib/modules/$kver for kernel/initramfs

lib/sysroot: Support /usr/lib/modules/$kver for kernel/initramfs

This is the new Fedora kernel standard layout; it has the advantage
of being in `/usr` like `/usr/lib/ostree-boot`, but it's not OSTree
specific.

Further, I think in practice forcing tree builders to compute the checksum is an
annoying stumbling block; since we already switched to e.g. computing checksums
always when doing pulls, the cost of doing another checksum for the
kernel/initramfs is tiny. The "bootcsum" becomes more of an internal
implementation detail.

Now, there is a transition; my current thought for this is that rpm-ostree will
change to default to injecting into both `/usr/lib/ostree-boot` and
`/usr/lib/modules`, and stop doing `/boot`, then maybe next year say we drop the
`/usr/lib/ostree-boot` by default.

A twist here is that the default Fedora kernel RPM layout (and what's in
rpm-ostree today) includes a kernel but *not* an initramfs in
`/usr/lib/modules`. If we looked only there, we'd just find the kernel. So we
need to look in both, and then special case this - pick the legacy layout if we
have `/usr/lib/modules` but not an initramfs.

While here, rework the code to have an `OstreeKernelLayout` struct which makes
dealing with all of the variables nicer.

Closes: #1079
Approved by: jlebon