tools/hotplug: cleanup Makefiles

tools/hotplug: cleanup Makefiles

Remove "build" targets.

Use simply expanded variables when recursively expanded variable
aren't needed. (Use ":=" instead of "=".)

Don't check if a directory already exist when installing, just create
it.

Fix $(HOTPLUGPATH), it shouldn't have any double-quote.

Some reindentation.

FreeBSD, "hotplugpath.sh" is already installed by common/.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>