kernel: add OPENEULER_VERSION_CODE to version.h

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7LEXY

---------------------------------------------

Add OPENEULER_VERSION_CODE to version.h, so modules or applications
can identify the specific kernel version.

In most cases, the out-of-tree module needs to identify the release
version of the openEuler for interface adaptation. The existing
OPENEULER_VERSION() and OPENEULER_VERSION_CODE() cannot distinguish
between LTS versions and innovative versions. Therefore, a new macro
OPENEULER_LTS is introduced.

The version information is generated in "linux/version.h", as bellow:
include/generated/uapi/linux/version.h

```
 #define OPENEULER_LTS 0          // 1: LTS; 0: none LTS
 #define OPENEULER_MAJOR 9999     // OS Major version, 9999: none released
 #define OPENEULER_MINOR 0        // SP release version
 #define OPENEULER_VERSION(a,b) (((a) << 8) + (b))
 #define OPENEULER_VERSION_CODE 2559744
```

Use Makefile.oever to avoid future merge conflicts.

Link:https://gitee.com/openeuler/kernel/issues/I575IT
Link:https://gitee.com/openeuler/kernel/issues/I7761D
Link:https://gitee.com/openeuler/kernel/pulls/1442
Link:https://gitee.com/openeuler/kernel/pulls/1219
Link:https://gitee.com/openeuler/kernel/pulls/195


Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Wei Li <liwei391@huawei.com>