lib/sysroot-deploy: Refactor kernel layout parsing

lib/sysroot-deploy: Refactor kernel layout parsing

I'd like to move the new canonical kernel directory to `/usr/lib/modules/$kver`,
as Fedora has done. The `get_kernel_from_tree()` function now abstracts over
parsing the data (src vs destination filenames, as well as checksum) in
preparation for adding the new case.

In preparation for this, let's change the current test suite to use the
*current* directory of `/usr/lib/ostree-boot`, and also add coverage of `/boot`.

Closes: #1053
Approved by: jlebon