Fix tinydtls implementation of sign & verify

Fix tinydtls implementation of sign & verify

Change-Id: Id9b721a12e517fb0fe5adae76b380acb04b467cd
Signed-off-by: Steve Clark <steve.clark@atmel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/3417
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Sachin Agrawal <sachin.agrawal@intel.com>
(cherry picked from commit 28fd8b67657cdbe3860302a3ea92d83515c1a688)
Reviewed-on: https://gerrit.iotivity.org/gerrit/3435
(cherry picked from commit f2da5d0b80c95fac89caae6dcc9b3d567ea82c8f)
Reviewed-on: https://gerrit.iotivity.org/gerrit/3443