readme: add Zephyr SDK build instructions
readme: add Zephyr SDK build instructions

Change-Id: Ia2fd236a1c2225413ce2b6e5f208ef61938d7eb6
Signed-off-by: Javier B Perez <dbb8a7ad4da91c8294f33dd484bd91c55a16d5ab@intel.com>

zephyrproject-rtos/meta-zephyr-sdk,zephyrproject-rtos/meta-zephyr-sdk,zephyrproject-rtos/meta-zephyr-sdk,zephyrproject-rtos/meta-zephyr-sdk