build: rename aarch64 target to arm64

Rename aarch64 build targets to arm64.  The gcc toolchain is still
aarch64.

Change-Id: Ia92d8a50824e5329cf00fd6f4f92eae112b7f3a3
diff --git a/target/board/generic_arm64/system.prop b/target/board/generic_arm64/system.prop
new file mode 100644
index 0000000..aee6b13
--- /dev/null
+++ b/target/board/generic_arm64/system.prop
@@ -0,0 +1,6 @@
+#
+# system.prop for generic arm64 sdk
+#
+
+rild.libpath=/system/lib/libreference-ril.so
+rild.libargs=-d /dev/ttyS0