cmd: Add pause command

This command is being introduced with the goal of allowing user-friendly
"generic use case" U-Boot builds to pause until user input under some
situations.

The main use case would be when a boot failure happens, to pause until
the user has had time to acknowledge the current state.

Tested using:

    make && ./u-boot -v -T -c 'ut lib lib_test_hush_pause'

Signed-off-by: Samuel Dionne-Riel <samuel@dionne-riel.com>
Cc: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 4a0c51c..e6a1718 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -96,6 +96,7 @@
 CONFIG_CMD_EFIDEBUG=y
 CONFIG_CMD_RTC=y
 CONFIG_CMD_TIME=y
+CONFIG_CMD_PAUSE=y
 CONFIG_CMD_TIMER=y
 CONFIG_CMD_SOUND=y
 CONFIG_CMD_QFW=y