initramfs-init should wait device shows up then mount
Boot alpine from usb in server may run into this problem https://github.com/alpinelinux/mkinitfs/blob/master/initramfs-init.in\#L487 . Mount failed, because usb is detect async. There is a rootwait kernel param [rootwait](https://github.com/torvalds/linux/blob/728254541ebcc7fee869c3c4c3f36f96be791edb/Documentation/admin-guide/kernel-parameters.txt\#L4172) to control this, alpine should do this too.
(from redmine: issue id 10635, created on 2019-06-29)