ARM: qemu-arm: enable USB boot in distro boot with UEFI

With this patch which adds a removable USB mass storage to a list
of bootable devices, USB boot is supported in distro boot if UEFI is
configured.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
diff --git a/include/configs/qemu-arm.h b/include/configs/qemu-arm.h
index 0431a79..65fdb1e 100644
--- a/include/configs/qemu-arm.h
+++ b/include/configs/qemu-arm.h
@@ -25,6 +25,7 @@
 #define CONFIG_ENV_SIZE			SZ_256K
 
 #define BOOT_TARGET_DEVICES(func) \
+	func(USB, usb, 0) \
 	func(SCSI, scsi, 0) \
 	func(VIRTIO, virtio, 0) \
 	func(DHCP, dhcp, na)