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.
Look more:
https://www.syslinux.org/archives/2012-December/019143.html
https://github.com/geneC/syslinux/blob/2ea44cbedb297bd6b409d5c1e0402d5f89592be4/core/elflink/load\_env32.c\#L167-L169
(from redmine: issue id 10614, created on 2019-06-24, closed on 2019-06-24)