Fix warning of "overriding commands for target ...boot.oat".

There seems to be bug in Kati that when we write it as:
LIBART_TARGET_BOOT_ART_EXTRA_FILES += boot.oat $(patsubst
%.art,%.oat,$(LIBART_TARGET_BOOT_ART_EXTRA_FILES))

boot.oat ends up in the value twice.
This works around the bug, filed as bug/27676615.

Bug: 27546601
Change-Id: Ifc330280496083a4c817e53bc5c77c86e48a2426
diff --git a/core/dex_preopt_libart.mk b/core/dex_preopt_libart.mk
index 05f2118..0429b62 100644
--- a/core/dex_preopt_libart.mk
+++ b/core/dex_preopt_libart.mk
@@ -71,8 +71,8 @@
 # is converted into to boot.art (to match the legacy assumption that boot.art
 # exists), and the rest are converted to boot-<name>.art.
 # In addition, each .art file has an associated .oat file.
-LIBART_TARGET_BOOT_ART_EXTRA_FILES := $(foreach jar,$(wordlist 2,999,$(LIBART_TARGET_BOOT_JARS)),boot-$(jar).art)
-LIBART_TARGET_BOOT_ART_EXTRA_FILES += boot.oat $(patsubst %.art,%.oat,$(LIBART_TARGET_BOOT_ART_EXTRA_FILES))
+LIBART_TARGET_BOOT_ART_EXTRA_FILES := $(foreach jar,$(wordlist 2,999,$(LIBART_TARGET_BOOT_JARS)),boot-$(jar).art boot-$(jar).oat)
+LIBART_TARGET_BOOT_ART_EXTRA_FILES += boot.oat
 
 my_2nd_arch_prefix :=
 include $(BUILD_SYSTEM)/dex_preopt_libart_boot.mk