main: copy u-boot files to working directory (MR 16)

U-boot files may be used to generate boot images during boot-deploy stage,
for example u-boot's FIT images, android boot images. If there's no u-boot
directory for device, skip copying.

deviceinfo: add UbootBoardname

Alpine u-boot package places board files in separate directories,
/usr/share/u-boot/$boardname. This param specifies device u-boot subdirectory.

pkgs/deviceinfo/deviceinfo.go

@@ -16,6 +16,7 @@ import (
 type DeviceInfo struct {
 	AppendDtb                     string
 	Arch                          string
+	UbootBoardname                string
 	BootimgAppendSEAndroidEnforce string
 	BootimgBlobpack               string
 	BootimgDtbSecond              string