Add generic aarch64 board config and build rules.

Change-Id: I8b4a377596705dfa0a3bd234162d183ec2ae9530
diff --git a/target/board/generic_aarch64/system.prop b/target/board/generic_aarch64/system.prop
new file mode 100644
index 0000000..29e4be9
--- /dev/null
+++ b/target/board/generic_aarch64/system.prop
@@ -0,0 +1,6 @@
+#
+# system.prop for generic aarch64 sdk
+#
+
+rild.libpath=/system/lib/libreference-ril.so
+rild.libargs=-d /dev/ttyS0