use relative path for true(1)

On FreeBSD true(1) is at /usr/bin/true.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>

xen/Makefile

@@ -102,7 +102,7 @@ $(TARGET): delete-unfresh-files
 	[ -e include/asm ] || ln -sf asm-$(TARGET_ARCH) include/asm
 	[ -e arch/$(TARGET_ARCH)/efi ] && for f in boot.c runtime.c compat.c efi.h;\
 		do ln -nsf ../../../common/efi/$$f arch/$(TARGET_ARCH)/efi/; done;\
-		/bin/true
+		true
 	$(MAKE) -f $(BASEDIR)/Rules.mk -C include
 	$(MAKE) -f $(BASEDIR)/Rules.mk -C arch/$(TARGET_ARCH) asm-offsets.s
 	$(MAKE) -f $(BASEDIR)/Rules.mk include/asm-$(TARGET_ARCH)/asm-offsets.h