Enable stack overflow checking on all context switches

Changes somewhere in the recent past have caused I2C operations to
consume more stack space.  The current failure mode is that after some
debug command or infrequent battery operation, the system fails.

Clean up and enable stack overflow detection by default, and add a
debug command (disabled by default) to verify overflow detection
works.

This adds several instructions to each context switch, but it's still
fairly inexpensive, and represents only a few percent increase in the
size of svc_handler().  That's better than silent failures.

BUG=chrome-os-partner:23938
BRANCH=none
TEST=Enable CONFIG_CMD_STACKOVERFLOW, then run the 'stackoverflow' command.
     This should cause a stack overflow to be detected in the CONSOLE task.

Change-Id: I9303aee5bd9318f1d92838b399d15fb8f6a2bbf9
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/176113
Reviewed-by: Bill Richardson <wfrichar@chromium.org>

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