sandbox: enable bootmenu command

Provide the bootmenu command on the sandbox for testing.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index fe8ea46..c509a92 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -39,6 +39,7 @@
 CONFIG_CMD_BOOTM_PRE_LOAD=y
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_BOOTEFI_HELLO=y
+CONFIG_CMD_BOOTMENU=y
 CONFIG_CMD_ABOOTIMG=y
 # CONFIG_CMD_ELF is not set
 CONFIG_CMD_ASKENV=y