Add system_id attribute to targets. This ID is supposed unique for a
combination of hardware, kernel, and the file system, and contains
elements from each.
1. Hardware is identified by the concatenation of MAC addresses of
'link/ether' network interfaces on the system. This method is used,
as DMI tables are often unimplemented on ARM targets.
2. The kernel is identified by its version.
3. The file system is identified by the concatenation of UUID's of the
target's partitions. It would be more correct to only use UUID of
the root partition, as system_id is not intended to be affected by
removable, media, however, there is no straight-forward way of
reliably identifying that without root.
system_id is intended to be used as an key for the purposes of caching
information about a particular device (e.g. so that it does not need to
be probed on each run).
Add two new methods to target that allow querying values of all sysfs
nodes in a sub-directory structure all at once. The difference is that
read_tree_values_flat returns a flat dict of path->value mappings;
read_tree_values returns a nested dict structure that mimics the
scanned sub-directories tree.
