sandbox: add rtc command to defconfigs

In order to allow adding unit tests of the rtc command, add it to the
various sandbox defconfigs.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
diff --git a/configs/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index dfcc435..4158b9b 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -49,6 +49,7 @@
 CONFIG_CMD_DNS=y
 CONFIG_CMD_LINK_LOCAL=y
 CONFIG_CMD_EFIDEBUG=y
+CONFIG_CMD_RTC=y
 CONFIG_CMD_TIME=y
 CONFIG_CMD_TIMER=y
 CONFIG_CMD_SOUND=y