bootspec: store 'root' field in each bootspec entry we load

bootspec: store 'root' field in each bootspec entry we load

This 'root' field contains the root path of the partition we found the
snippet in. The 'kernel', 'initrd', 'efi', … fields are relative to this
path.

This becomes particularly useful later when we add support for loading
snippets from both the ESP and XBOOTLDR, but already simplifies the code
for us a bit in systemctl.