alpine.mk: use /dev/usbdisk rather than /dev/sda1 for usb images

The sda1 might be a sata or scsi harddisk so usb image might be sdb1 or sdc1.
The /dev/usbdisk link will always point to the first usb disk.

alpine.mk

@@ -173,7 +173,7 @@ $(SYSLINUX_CFG):
 	@echo "default $(KERNEL_NAME)" >>$@
 	@echo "label $(KERNEL_NAME)" >>$@
 	@echo "	kernel /boot/$(KERNEL_NAME)" >>$@
-	@echo "	append initrd=/boot/$(KERNEL_NAME).gz alpine_dev=sda1:vfat modules=sd-mod,usb-storage quiet" >>$@
+	@echo "	append initrd=/boot/$(KERNEL_NAME).gz alpine_dev=usbdisk:vfat modules=sd-mod,usb-storage quiet" >>$@
 
 ISO_KERNEL	:= $(ISO_DIR)/boot/$(KERNEL_NAME)
 ISO_REPOS	:= $(addprefix $(ISO_PKGDIR)/,$(REPOS))