uroot package: add : syntax back for extra files
uroot package: add : syntax back for extra files

The : syntax lets you do stuff like this:
-files /path/to/prototype/root:etc

It will source from /path/to/prototype/root/etc but
put it into the cpio archive as etc. It's impossible to
live without, I keep learning.

I used SplitN, not Split, as there are valid paths (maybe?)
with : in them. That said, we may find we want a more robust
way to do this.

Signed-off-by: Ronald G. Minnich <f569d0af9a3481c4007b95c7180d6c41009f2513@gmail.com>

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