This closes #5.

Merge remote-tracking branch 'rsalveti/zephyr-updates'

* rsalveti/zephyr-updates:
  zephyr: use SYS_LOG instead of printk
  image_rsa: include missing string.h
  boot/zephyr/prj.conf: disable bluetooth support by default
  boot/zephyr/prj.conf: remove SOC_FLASH_STM32F4, make it generic
  zephyr: targets: add support for 96b_nitrogen
  zephyr: restructure the build process to use board config files