u-boot.mk: add LOCALVERSION (explicitly specify OpenWrt build)

u-boot.mk: add LOCALVERSION (explicitly specify OpenWrt build)

For debugging purposes, we need to know if users are using modified
U-boot versions or not. Currently, the U-boot version is somehow
stripped. This is a little bit problematic when there are
backported/wip/to-upstream patches.

To make it more confusing, there was (before this commit) two U-boot
versioning. U-boot compiled by OpenWrt build bots are missing ``Build:``
This is also the case when the U-boot is compiled locally.

Example:
```
U-Boot SPL 2022.01 (Jan 27 2022 - 00:24:34 +0000)
U-Boot 2022.01 (Jan 27 2022 - 00:24:34 +0000)
```

On the other hand, if you run full build, you can at least see, where it
was compiled. Notice added ``Build:``.

Example:
```
U-Boot 2022.01 (Jan 27 2022 - 00:24:34 +0000), Build: jenkins-turris-os-packages-burstlab-omnia-216
```

In both cases, it is not clear to U-boot developers if it is an unmodified
build. This is also caused that there is a missing ``.git`` file from
U-boot folder, and so there is no history. It leads to that it can not
contain suffix ``-dirty`` (uncommitted modifications) or even something
else like number of commits, etc. [1]

When U-boot is compiled as it should be, the version should look like
this: ``U-Boot 2022.04-rc1-01173-g278195ea1f (Feb 11 2022 - 14:46:50 +0100)``
The date is not changed daily when there are new OpenWrt builds.

This commit adds OpenWrt specific version, which could be verified by
using strings.

```
$ strings bin/targets/mvebu/cortexa9/u-boot-omnia/u-boot-spl.kwb | grep -E "OpenWrt*"
U-Boot SPL 2022.01-OpenWrt-r18942+54-cbfce92367 (Feb 21 2022 - 13:17:34 +0000)
arm-openwrt-linux-muslgnueabi-gcc (OpenWrt GCC 11.2.0 r18942+54-cbfce92367) 11.2.0
2022.01-OpenWrt-r18942+54-cbfce92367
U-Boot 2022.01-OpenWrt-r18942+54-cbfce92367 (Feb 21 2022 - 13:17:34 +0000)
```

[1] https://u-boot.readthedocs.io/en/latest/develop/version.html

Reported-by: Pali Rohár <pali@kernel.org>
Suggested-by: Karel Kočí <karel.koci@nic.cz>
Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>