lib/deploy: Add .img to end of initramfs in /usr/lib/modules

Follow up to <https://github.com/ostreedev/ostree/pull/1079>; I was working on
the rpm-ostree updates for this, and I think it's more consistent if we have
`.img` here, since that's a closer match to the "remove $kver" that results in
`vmlinuz`. Also just best practice to have file suffix types where they make
sense.

The astute reader might notice this sneaks in a change where we'd crash if the
legacy bootdir didn't have an initramfs...yeah, should probably have test
coverage of that.

Closes: #1095
Approved by: jlebon

docs/manual/deployment.md

@@ -45,7 +45,7 @@ distinguish it from the concept of a deployment.
 
 First, the tree must include a kernel (and optionally an initramfs).  The
 current standard locations for these are `/usr/lib/modules/$kver/vmlinuz` and
-`/usr/lib/modules/$kver/initramfs`.  The "boot checksum" will be computed
+`/usr/lib/modules/$kver/initramfs.img`.  The "boot checksum" will be computed
 automatically.  This follows the current Fedora kernel layout, and is
 the current recommended path.  However, older versions of libostree don't
 support this; you may need to also put kernels in the previous (legacy)