edtlib: fix "last modified" semantic for included property specs

Although the PropertySpec.path attribute is documented as
"the file where the property was last modified",
all property specs in Binding.prop2specs will claim
they were last modified by the top-level binding itself.

Consider:
- I1 is a base binding that specifies properties x and y
- I2 is an "intermediate" binding that includes I1,
  modifying the specification for property x
- B is a top-level bindings that includes I2,
  and specifies an additional property p

When enumerating the properties of B,
we expect the values of PropertySpec.path to tell us:
- y was last modified by I1
- x was last modified by I2
- p was last modified by B

However, the Binding constructor:
- first merges all included bindings into the top-level one
- eventually initializes specifications for all the defined properties

As a consequence, all defined properties claim they were last modified
by the top-level binding file.

We should instead:
- first, take into account their own specifications for the
  included properties
- eventually update these specifications with the properties
  the top-level binding adds or modifies

Signed-off-by: Christophe Dufaza <chris@openmarl.org>