Some more build-system finessing:

Some more build-system finessing:
 1. subdirs-y -> subdir-y (match Linux name, and also obj-y
    is singular so this makes sense).
 2. subdirs can be declared with or without trailing slash
 3. As with Linux, subdirs can be declared in the obj-y list
    but they must be distinguished by a trailing slash

Signed-off-by: Keir Fraser <keir@xensource.com>