tree: Makefiles: syntax, convert all ${VAR} to $(VAR)

This is purely personal preference.  Most of the Makefiles use $() for
Makefile variables, but a couple of places use ${}.  Since this obscured
some later Makefile changes I figured I'd just make them all the same up
front.

Signed-off-by: Eric Paris <[email protected]>
Acked-by: Dan Walsh <[email protected]>
20 files changed