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_defconfig b/configs/sandbox_defconfig
index 8cf2ad5..e0dffa3 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -69,6 +69,7 @@
 CONFIG_CMD_BMP=y
 CONFIG_CMD_BOOTCOUNT=y
 CONFIG_CMD_EFIDEBUG=y
+CONFIG_CMD_RTC=y
 CONFIG_CMD_TIME=y
 CONFIG_CMD_TIMER=y
 CONFIG_CMD_SOUND=y