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/README.txt b/target/board/generic_arm64/README.txt
new file mode 100644
index 0000000..21c71d7
--- /dev/null
+++ b/target/board/generic_arm64/README.txt
@@ -0,0 +1,5 @@
+The "generic_arm64" product defines a non-hardware-specific arm64 target
+without a kernel or bootloader.
+
+It is not a product "base class"; no other products inherit
+from it or use it in any way.