boards: rv-virt: Update README.txt

Summary:
- This commit updates README.txt regarding how to build kernel
  and apps for BUILD_KERNEL
- Also update on parallel build for BUILD_FLAT

Impact:
- None

Testing:
- Tested with qemu-7.1.0

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

boards/risc-v/qemu-rv/rv-virt/README.txt

@@ -10,7 +10,7 @@
   $ make
   $ sudo make install
 
-3. Configure and build NuttX
+3.1. Configure and build NuttX for BUILD_FLAT
 
   $ mkdir ./nuttx; cd ./nuttx
   $ git clone https://github.com/apache/incubator-nuttx.git nuttx
@@ -18,7 +18,22 @@
   $ cd nuttx
   $ make distclean
   $ ./tools/configure.sh rv-virt:nsh
-  $ make
+  $ make V=1 -j7
+
+3.2 Configure and build NuttX for BUILD_KERNEL
+
+  $ mkdir ./nuttx; cd ./nuttx
+  $ git clone https://github.com/apache/incubator-nuttx.git nuttx
+  $ git clone https://github.com/apache/incubator-nuttx-apps.git apps
+  $ cd nuttx
+  $ make distclean
+  $ ./tools/configure.sh rv-virt:knsh64
+  $ make V=1 -j7
+  $ make export V=1
+  $ cd ../apps
+  $ ./tools/mkimport.sh -z -x ../nuttx/nuttx-export-*.tar.gz
+  $ make import V=1
+  $ cd ../nuttx
 
 4. Run the nuttx with qemu