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
1 file changed