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/sandbox64_defconfig b/configs/sandbox64_defconfig
index f82dc9c..2066d0a 100644
--- a/configs/sandbox64_defconfig
+++ b/configs/sandbox64_defconfig
@@ -7,6 +7,7 @@
 CONFIG_BOOTSTAGE_STASH_ADDR=0x0
 CONFIG_DEFAULT_DEVICE_TREE="sandbox64"
 CONFIG_SANDBOX64=y
+CONFIG_DEBUG_UART=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y