arm64: don't build emulator

There is no emulator kernel yet, so don't try to build the emulator.

Change-Id: I714f5717f439b4e48a973f4355f2c6684af79764
diff --git a/target/board/generic_arm64/BoardConfig.mk b/target/board/generic_arm64/BoardConfig.mk
index 72b21e6..0bdcd99 100644
--- a/target/board/generic_arm64/BoardConfig.mk
+++ b/target/board/generic_arm64/BoardConfig.mk
@@ -59,3 +59,5 @@
 # Temporarily disable linking against compiler_rt until there is an arm64
 # version
 WITHOUT_LIBCOMPILER_RT := true
+
+BUILD_EMULATOR := false