limits.h: add define for HOST_NAME_MAX

according to the rules defined here:
    http://pubs.opengroup.org/onlinepubs/009695399/basedefs/limits.h.html


add the definition for HOST_NAME_MAX to limits.h file,
and set the default value to _POSIX_HOST_NAME_MAX as 255

Change-Id: Iddd5c6c569f4e0a14994c7a7c54985f3e7809fc4
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>