* Add otPlatRadioGetReceiveSensitivity() API
* Add a new otPlatRadioGetReceiveSensitivity() API to get sensitivity value;
* Use the receive sensitivity value as the noise floor for link metric computation;
* Remove some unused noise floor related functions.
* Add SPINEL_PROP_PHY_RX_SENSITIVITY Spinel/NCP property
* change user mode openthread to enable dynamic memory allocation in mbedtls
* complete merge by adding OPENTHREAD_MULTIPLE_INSTANCE to last file
* remove some extraneous changes that got pulled in accidentally
* fix spacing
* fix crash in existing tests due to this change
* fix x86 build
* Fix scanning behavior with default channel. Change logic around otPlatRadioGetTransmitBuffer so that the buffer becomes the property of OT post call. The MAC will keep a pointer to the tx buffer and pass it into otPlatRadioTransmit. This change should make the notion of retrying the transmit operation with the same buffer more intuitive.