While BSD make support both $() and ${} around variables, the NetBSD
source tree uses only ${} by convention.
Imported software is left as is, and sometimes $() is used when the
containing Makefile/Makefile fragment is used both by GNU make and BSD
make, as it can happen for the tools, and other parts as well which are
compiled using the host make tool.
Change-Id: Ic7d480812fde53e7e3e95275a30a3b720c95cc15
