dhrystone: remove usage of FOO_DIR_PREFIX

As the semantics of FOO_DIR_PREFIX will change in the following patch, we
want to remove unnecessary usage of this variable.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/dhrystone/dhrystone.mk

@@ -10,7 +10,7 @@ DHRYSTONE_SITE = http://www.netlib.org/benchmark/
 
 define DHRYSTONE_EXTRACT_CMDS
 	(cd $(@D) && $(SHELL) $(DL_DIR)/$($(PKG)_SOURCE))
-	$(Q)cp $($(PKG)_DIR_PREFIX)/dhrystone/Makefile $(@D)/
+	$(Q)cp package/dhrystone/Makefile $(@D)/
 endef
 
 define DHRYSTONE_BUILD_CMDS