Zephyr 1.6.1 fixes the following issues:
- Makefile (arc/soc/quark_se): New compiler options
- Makefile (arc/soc/em*): New compiler options
- arc: add -fno-delete-null-pointer-checks
- Makefile.toolchain.zephyr: Modifications for SDK 0.9
- Bluetooth: ATT: Fix using k_fifo API with net_buf
- Bluetooth: fix write cmd handling
- Bluetooth: GATT: Fix missing connection address comparison
- arm: Cortex-M0: Adapt core register code to M0
- arm: nvic: Fix exception priority access on Cortex-M0(+)
- arm: nvic: Use uint8_t for priority level
- Update to tinycrypt 0.2.6
- Update to mbedTLS 2.4.2