live-server kernel changed to casper/vmlinuz

A couple of days ago, the live-server kernel changed from /casper/vmlinuz.efi to just /casper/vmlinuz. Tests started failing (https://platform-qa-jenkins.ubuntu.com/view/server/job/ubuntu-bionic-live-server-amd64-iso-static-validation/79/console):

DEBUG: bsdtar list command: bsdtar -t -v -f /data/iso/ubuntu-server/bionic-live-server-amd64.iso casper/vmlinuz.efi
bsdtar: casper/vmlinuz.efi: Not found in archive
bsdtar: Error exit delayed from previous errors.
ERROR: test_efi_secure_boot_signatures (__main__.TestValidateISO)
ERROR: Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "/usr/share/utah/isotest/iso_static_validation.py", line 508, in test_efi_secure_boot_signatures
    kernel = self.iso.extract(iso.kernelpath(), self.temp_dir)
  File "/usr/lib/python2.7/dist-packages/utah/iso.py", line 279, in extract
    cmd = self.getrealfile(filename)
  File "/usr/lib/python2.7/dist-packages/utah/iso.py", line 252, in getrealfile
    .format(filename, self.image, err))
UTAHISOException: Cannot list casper/vmlinuz.efi in /data/iso/ubuntu-server/bionic-live-server-amd64.iso: Command '['bsdtar', '-t', '-v', '-f', '/data/iso/ubuntu-server/bionic-live-server-amd64.iso', 'casper/vmlinuz.efi']' returned non-zero exit status 1

From IRC:
<slangasek> andreas: this is probably fallout from the changes to the kernel packaging, such that there is no longer a separate linux-signed image. I don't know why this caused the filename to change on the image
(...)
<slangasek> andreas: confirmed, the code in debian-cd is inconsistent in that it only copies filesystem.kernel-$FLAVOUR.efi.signed to vmlinuz.efi and filesystem.kernel-$FLAVOUR to vmlinuz which is logically inconsistent but doesn't impact the bootability
(...)
<andreas> so signed -> vmlinuz.efi, and unsigned -> vmlinuz
<andreas> and there is no signed now?
<slangasek> there is no separate file named .signed
<andreas> the .efi one was the signed one
<slangasek> it was. now vmlinuz is the signed one.
<slangasek> andreas: anyway, it's a bad test

utah/iso.py:446
    def kernelpath(self):
        kernelpath = './install/vmlinuz'
        if self.arch == 'ppc64el':
            kernelpath = './install/vmlinux'
        elif self.installtype == 'mini':
            self.logger.debug('Getting kernel for mini image')
            kernelpath = 'linux'
        elif self.installtype == 'live-server':
            self.logger.debug('Getting kernel for live-server')
            kernelpath = 'casper/vmlinuz.efi'
                          ^^^^^^^^^^^^^^^^^^