lito: Assign subsys nodes file contexts in a dynamic way.

The subsystem name to subsys number mapping is not constant
and can change based on the order of probing OR incase a new
subsystem gets added.
To handle such cases, this change assigns the contexts in a
more dynamic way using regex within file-contexts file.

Change-Id: Ibc688f334381dffec2bf5419305fabcf2ecd72e6