[sw/boot_rom] Rename "boot_rom" to "test_rom".

The ROM code in `sw/device/boot_rom/` is used purely for testing purposes,
and is not designed to be used as the production mask ROM. The mask ROM
code lives in `sw/device/silicon_creator/mask_rom`. To make this more clear,
the `sw/device/boot_rom` ia renamed and relocated to
`sw/device/lib/testing/test_rom`.

This fixes #9154.

Signed-off-by: Timothy Trippel <ttrippel@google.com>
diff --git a/sw/device/_index.md b/sw/device/_index.md
index cc8773f..8376865 100644
--- a/sw/device/_index.md
+++ b/sw/device/_index.md
@@ -26,7 +26,7 @@
 
 There are also prototype versions of some of the boot stages, now only used for testing:
 
-- [`sw/device/boot_rom`]({{< relref "sw/device/boot_rom/README.md" >}}) is a previous, testing-only version of the Mask ROM.
+- [`sw/device/lib/testing/test_rom`]({{< relref "sw/device/lib/testing/test_rom/README.md" >}}) is a previous, testing-only version of the Mask ROM.
 - `sw/device/exts` contains software for our prototype second boot stage images.
 
 ## Documentation Index