Add generic aarch64 board config and build rules.

Change-Id: I8b4a377596705dfa0a3bd234162d183ec2ae9530
diff --git a/envsetup.sh b/envsetup.sh
index 71b0ba7..2ae7cb8 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -132,6 +132,8 @@
             ;;
         arm) toolchaindir=arm/arm-linux-androideabi-$targetgccversion/bin
             ;;
+        aarch64) toolchaindir=aarch64/aarch64-linux-android-$targetgccversion/bin
+            ;;
         mips) toolchaindir=mips/mipsel-linux-android-$targetgccversion/bin
             ;;
         *)