configs: sandbox: activate command env select and env load

Add support of environment location with the new env command:
'env select' and 'env load'

The ENV backend is selected by priority order
- 0 = "nowhere" (default at boot)
- 1 = "EXT4"

To test EXT4 env support, this backend is selected by name:
> env select EXT4

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 09b6dcb..0fce901 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -36,6 +36,8 @@
 CONFIG_CMD_ENV_FLAGS=y
 CONFIG_CMD_NVEDIT_EFI=y
 CONFIG_CMD_NVEDIT_INFO=y
+CONFIG_CMD_NVEDIT_LOAD=y
+CONFIG_CMD_NVEDIT_SELECT=y
 CONFIG_LOOPW=y
 CONFIG_CMD_MD5SUM=y
 CONFIG_CMD_MEMINFO=y