Ubuntu enables GCC's stack protector by default (see
https://wiki.ubuntu.com/Security/Features). This causes link errors
like the following:
...undefined reference to `__stack_chk_fail'
To avoid these errors, this patch adds the "-fno-stack-protector" flag
to both the CFLAGS used by the Contiki build process and the CFLAGS
used by the newlib build process.