Documentation/rv-virt: Add entries for nsbi[|64] defconfigs

This commit adds entries for the `rv-virt:nsbi[|64]` defconfigs in
the `rv-virt` board documentation page.

Documentation/platforms/risc-v/qemu-rv/boards/rv-virt/index.rst

@@ -371,6 +371,39 @@ nsh64
 
 Identical to the `nsh`_ configuration, but for 64-bit RISC-V.
 
+nsbi
+----
+
+This is similar to the `knsh`, but using NuttX's native (minimalistic)
+SBI. It uses `hostfs` and QEMU in semi-hosting mode to load the
+user-space applications. This is intended for 32-bit RISC-V.
+
+To build it, use the following command::
+
+    $ make V=1 -j$(nproc)
+    $ make export V=1 -j$(nproc)
+    $ pushd ../apps
+    $ ./tools/mkimport.sh -z -x ../nuttx/nuttx-export-*.tar.gz
+    $ make import V=1 -j$(nproc)
+    $ popd
+
+Run it with QEMU using the default command for 32-bit RISC-V without
+the ``-bios none`` option. Please note that it still runs in S-mode,
+but bypasses QEMU's OpenSBI.
+
+In `nsh`, applications can be run directly::
+
+    nsh> hello
+
+nsbi64
+------
+
+Identical to the `nsbi`_ configuration, but for 64-bit RISC-V.
+
+Run it with QEMU using the default command for 64-bit RISC-V without
+the ``-bios none`` option. Please note that it still runs in S-mode,
+but bypasses QEMU's OpenSBI.
+
 smp
 ---