This weird error came up during some update.
Summary of the fix since i'm short on time...:
Probably you had used a block device path for /boot and this path changed. System can still come up fine, but /boot won't be mounted.
Extlinux is not able to work if /boot isn't mounted. This patch would possibly improve: http://www.syslinux.org/archives/2014-December/022987.html
Fix fstab, mount /boot and run apk fix.
Error appears in final stages of apk upgrade
apk fix cannot help you with this error, but at least it stays only one single error
lsscsi can be used to verify which block devices you have and which one you need
check partitions on suspected boot device (which is also the one where / is mounted from)
---- and now fix your /etc/fstab
And now, if you mount /boot, the apk fix should work and update your initramfs.