libxml2: add flag to exclude lib

It has been reported that this file causes bootloops
on some device(s)

Usage:
TW_EXCLUDE_LIBXML2 := true

Change-Id: I426536cc143b758f6821e5d2fc10ce942c3b1c79
diff --git a/prebuilt/Android.mk b/prebuilt/Android.mk
index bb7f89a..aed16e2 100755
--- a/prebuilt/Android.mk
+++ b/prebuilt/Android.mk
@@ -197,7 +197,9 @@
         RECOVERY_LIBRARY_SOURCE_FILES += $(TARGET_OUT_SHARED_LIBRARIES)/android.hardware.vibrator-V1-cpp.so
         RECOVERY_LIBRARY_SOURCE_FILES += $(TARGET_OUT_SHARED_LIBRARIES)/libstatslog.so
         RECOVERY_LIBRARY_SOURCE_FILES += $(TARGET_OUT_SHARED_LIBRARIES)/libsoft_attestation_cert.so
-        RECOVERY_LIBRARY_SOURCE_FILES += $(TARGET_OUT_SHARED_LIBRARIES)/libxml2.so
+        ifneq ($(TW_EXCLUDE_LIBXML2), true)
+            RECOVERY_LIBRARY_SOURCE_FILES += $(TARGET_OUT_SHARED_LIBRARIES)/libxml2.so
+        endif
 
         ifneq ($(wildcard system/keymaster/keymaster_stl.cpp),)
             RECOVERY_LIBRARY_SOURCE_FILES += $(TARGET_OUT_SHARED_LIBRARIES)/libkeymaster_portable.so