Initial Zephyr support

[hathach]

Add initial support for Zephyr

• Add osal_zephyr.h and OPT_OS_ZEPHYR
• update dcd_nrf5x.c to compile with zephyr
• Rework cmake build system to work with zephyr package and west: supported examples are board_test, cdc_msc, msc_dual_lun. -DRTOS=zephyr need to passed to cmake/west to tell example cmake to use zephyr package
• make won't be supported, since zephyr based entirely on cmake and rather complicated
• only support/test with pca10056(nrf52840)
• examples code does not make use of zephyr API just yet, therefore led may not blink correctly, cdc is not tested, however with both cdc_msc and msc_dual_lun, thumbdrive is showed up as proof of concept.
• other ports such as samd may need some twweak and API/include usage to compatible with asf/sdk used by zephyr, but this is totally possible.

to build zephyr, first install west and zephyr sdk then cd to tinyusb repo then init workspace using examples/west.yml and fetch zephyr-os with

west init -l examples # init zephyr project 
west update

Then build with west, don't forget to have -- -DRTOS=zephyr for tinyusb example's CMakeLists.txt to fetch zephyr package

west build -b pca10056 -d examples/device/cdc_msc/build examples/device/cdc_msc -- -DRTOS=zephyr
west build -b pca10056 -d examples/device/msc_dual_lun/build examples/device/msc_dual_lun -- -DRTOS=zephyr

@tannewt @ladyada