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>
2 files changed