doc: add bootelf command documentation

Signed-off-by: Maxim Moskalets <maximmosk4@gmail.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 2f211f7..49b354e 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -33,6 +33,7 @@
    cmd/bootd
    cmd/bootdev
    cmd/bootefi
+   cmd/bootelf
    cmd/bootflow
    cmd/booti
    cmd/bootm