[cmake] set OT_MBEDTLS to OT_EXTERNAL_MBEDTLS at top-level (#1)
[cmake] set OT_MBEDTLS to OT_EXTERNAL_MBEDTLS at top-level (#1)

This is needed because both `sleepy-demo-mtd` and `sleepy-demo-ftd`
link against `${OT_MBEDTLS}`. Without this change, OT_MBEDTLS is not
defined in src/sleepy-demo/CMakeLists.txt
openthread/ot-efr32,openthread/ot-efr32,openthread/ot-efr32