Resurrect generic_armv5

But only for unbundled build.
Add it to tapas arch options.

Bug: 8108463
Change-Id: I53cd63c6ab2113fee3992650b561658231ea9921
diff --git a/target/board/generic_armv5/README.txt b/target/board/generic_armv5/README.txt
new file mode 100644
index 0000000..25d590a
--- /dev/null
+++ b/target/board/generic_armv5/README.txt
@@ -0,0 +1,5 @@
+The "generic_armv5" product defines a non-hardware-specific 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.