uroot package: fix: syntax for extra files
uroot package: fix: syntax for extra files

The previous commit b7cae605bd439eeef5c169a6f65819ddae3d6e55 introduced
a regression where using the old syntax (e.g. `-files something.txt`) is
truncating the first character (e.g. the resulting rootfs will contain
"omething.txt"). This diff avoids the truncation when the old syntax is
used.

GanShun/u-root,hugelgupf/u-root,GanShun/u-root,hugelgupf/u-root,rminnich/u-root,rminnich/u-root,u-root/u-root,u-root/u-root,GanShun/u-root,hugelgupf/u-root,rminnich/u-root,u-root/u-root,rminnich/u-root,hugelgupf/u-root