Docs: Update run commands for new build system

Update docs on run commands for using the new build system.
 - Image name for mcuboot is changed to 'bl2'.
 - Images are put under <build_dir>.

Change-Id: I45f56abd66dde92a046fe97a3597337eadafb70d
Signed-off-by: Summer Qin <summer.qin@arm.com>
diff --git a/docs/getting_started/tfm_user_guide.rst b/docs/getting_started/tfm_user_guide.rst
index 8386eab..a4f5822 100644
--- a/docs/getting_started/tfm_user_guide.rst
+++ b/docs/getting_started/tfm_user_guide.rst
@@ -49,7 +49,7 @@
 ------------------------------------------------------------
 To test TF-M with bootloader, one must apply the following changes:
 
-- Add ``mcuboot.axf`` to symbol files in DS-5 in Debug Configuration
+- Add ``bl2.axf`` to symbol files in DS-5 in Debug Configuration
   menu.
 - Replace the last two lines of the previous command with this: