Boot fails with alpine-vanilla-3.1.0_rc4-x86_64.iso

Trying to boot from that iso gives an error that '/boot/vmlinuz-vanilla cannot be found' (right after boot: prompt).
Looking inside the .iso there is indeed no /boot/vmlinuz-vanilla, but instead 2 files, one named boot/vmlinuz, one /boot/vanilla. Both have the same size.
I renamed vmlinuz to vmlinuz-vanilla, but then after boot: prompt the message is 'i8042: No controller found'

Update: after a few seconds the boot sequence starts nevertheless, and it looks successful.
Any idea where the message comes from and what it means? Thanks.

Update 2: setup-bootable eventually fails with ERROR: unsatisfiable constraint: syslinux (missing): required by: world [syslinux]
Obviously, installation of syslinux failed. Additionally, I've noticed that folder /usr/share contains only a single folder, udhcpc, no syslinux or anything else.
Also tried with alpine-vanilla-3.0.6-x86_64.iso, but same result.
Is this related to the first message 'i8042: No controller found' or is anything else wrong here?