tools: eliminate LIBXL_BLKTAP2

Use CONFIG_BLKTAP2 directly. There is no reason why one would want to
set LIBXL_BLKTAP2 separately as things stand.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>

tools/Rules.mk

@@ -163,9 +163,7 @@ else
 CFLAGS += -O2 -fomit-frame-pointer
 endif
 
-LIBXL_BLKTAP ?= $(CONFIG_BLKTAP2)
-
-ifeq ($(LIBXL_BLKTAP),y)
+ifeq ($(CONFIG_BLKTAP2),y)
 CFLAGS_libblktapctl = -I$(XEN_BLKTAP2)/control -I$(XEN_BLKTAP2)/include $(CFLAGS_xeninclude)
 SHDEPS_libblktapctl =
 LDLIBS_libblktapctl = $(SHDEPS_libblktapctl) $(XEN_BLKTAP2)/control/libblktapctl$(libextension)