tools/turbostat: allow user to alter DESTDIR and PREFIX

When run
	make -C tools DESTDIR=/my/nice/dir turbostat_install
get a message
	install: cannot create regular file '/usr/bin/turbostat': Permission denied

Allow user to alter DESTDIR and PREFIX variables.
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

tools/power/x86/turbostat/Makefile

 CC		= $(CROSS_COMPILE)gcc
 BUILD_OUTPUT	:= $(CURDIR)
-PREFIX		:= /usr
-DESTDIR		:=
+PREFIX		?= /usr
+DESTDIR		?=
 
 ifeq ("$(origin O)", "command line")
 	BUILD_OUTPUT := $(O)