Syslinux should use /boot/syslinux instead /boot
All the scripts eg update-extlinux installing extlinux into /boot
This is kindly wrong because syslinux is searching ldlinux.c32 in
"/boot/syslinux", "/syslinux", "/"
but not in /boot
It will work fine if /boot located on separated partition, but it will not work if you would try to install alpine linux using single partition.
(from redmine: issue id 10614, created on 2019-06-24, closed on 2019-06-24)