version: add version macro

Use macro for checking in the code for a specific version,
for example:

 if ZEPHYR_VERSION_CODE >= ZEPHYR_VERSION(0,6,2)
...
 endf

Change-Id: I21f152ca3444630a0273e4a8115b56a2a4179924
Signed-off-by: Anas Nashif <anas.nashif@intel.com>