01fips: fix HMAC file path resolution

01fips: fix HMAC file path resolution

There is a small regression in #343: when handling the 'separate boot
partition' case, we're checking for the kernel image in the wrong
location: `BOOT_IMAGE` is the `/boot`-relative path to the kernel image,
so `/boot/${BOOT_IMAGE_PATH}/${BOOT_IMAGE}` expands to e.g.
`/boot/mysubdir1/mysubdir2/mysubdir1/mysubdir2/vmlinuz...`.

We should be using `BOOT_IMAGE_NAME` here instead (and in fact, the next
if-statement does this correctly, so it might've just been accidentally
left out of #343).