configs: sandbox: activate DEBUG_UART

Add CONFIG_DEBUG_UART=y for all sandbox defconfig
as it is already done in sandbox_defconfig.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index edca7f1..efd99a9 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -5,6 +5,7 @@
 CONFIG_ENV_SIZE=0x2000
 CONFIG_BOOTSTAGE_STASH_ADDR=0x0
 CONFIG_DEFAULT_DEVICE_TREE="sandbox"
+CONFIG_DEBUG_UART=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y