Support subname= to change name of sub Blueprints files

Setting the "subname" variable will modify the name of Blueprints
files searched in sub directories.  If the specified name is not
found it will fall back to trying "Blueprints".  The subname
variable stays in scope for all subdirectories.

Change-Id: If3c8ac54735042076fb264d33719d9ea4b8934b3