add support to boot from usb

alpine.mk

@@ -249,7 +249,7 @@ $(SYSLINUX_CFG):
 	@echo "default $(KERNEL_NAME)" >>$@
 	@echo "label $(KERNEL_NAME)" >>$@
 	@echo "	kernel /boot/$(KERNEL_NAME)" >>$@
-	@echo "	append initrd=/boot/$(KERNEL_NAME).gz alpine_dev=sda1:vfat quiet" >>$@
+	@echo "	append initrd=/boot/$(KERNEL_NAME).gz alpine_dev=sda1:vfat modules=usb-storage,sd-mod quiet" >>$@
 
 ISO_KERNEL	:= $(ISO_DIR)/boot/$(KERNEL_NAME)
 ISO_PKGDIR	:= $(ISO_DIR)/packages

initramfs-init

@@ -133,8 +133,14 @@ mkdir -p $ALPINE_MNT
 if [ -n "$ALPINE_DEV_FS" ]; then
 	mount_opts="-t $ALPINE_DEV_FS"
 fi
-mount $mount_opts /dev/$ALPINE_DEV $ALPINE_MNT >/dev/null 2>&1
+
+# usb might need some time to settle so we retry a few times
+for i in $(seq 0 19); do
+	mount $mount_opts /dev/$ALPINE_DEV $ALPINE_MNT >/dev/null 2>&1 && break
+	sleep 1
+done
 eend $?
+
 ebegin "Mounting loopback device for kernel modules"
 modprobe loop
 if [ -n "$KOPT_modloop" ]; then