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/sandbox64_defconfig b/configs/sandbox64_defconfig
index ac604b5..dcf2f44 100644
--- a/configs/sandbox64_defconfig
+++ b/configs/sandbox64_defconfig
@@ -60,6 +60,7 @@
 CONFIG_CMD_ETHSW=y
 CONFIG_CMD_BMP=y
 CONFIG_CMD_EFIDEBUG=y
+CONFIG_CMD_RTC=y
 CONFIG_CMD_TIME=y
 CONFIG_CMD_TIMER=y
 CONFIG_CMD_SOUND=y