version: Update version handling and code

The kernel version has been converted from a string to a four-byte
quantity that is divided into two parts.
Part 1: The three most significant bytes represent the kernel's
numeric version, x.y.z. These fields denote:
      x -- major release
      y -- minor release
      z -- patchlevel release
Each of these elements must therefore be in the range 0 to 256, inclusive.

Part 2: The least significant byte is reserved for future use.

EXTRAVERSION is used for intermediate releases and customised
builds on top of an existing/released version.

Change-Id: Ibb7dd5654c36daeaa3a53499857d254aaef25bdf
Signed-off-by: Anas Nashif <anas.nashif@intel.com>