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.
An earlier revision of this patch had test for invalid lines, but I
decided to remove it because deviceinfo is sourced by so many things in
pmOS that it should be /very/ obvious that there are invalid lines or
other garbage in the file.
I also tried to make the parser fairly forgiving if it does encounter
things that don't meet the deviceinfo "spec", so that mkinitfs will try
as hard as it can to make an initfs.
