What it does?
------------
This depends on uboot-mvebu, which is located in OpenWrt main repo. It
compiles the U-boot image for Turris Omnia router by using ``UBOOT_CONFIG``,
which is defined in the ``include/u-boot.mk``, where it is used in
``Build/Configure/U-Boot``. Because of ``Build/InstallDev`` the U-boot
binary is copied to staging_dir, which we copy and package it to the
standalone package as we want to provide users via opkg.

Why do it like this?
--------------------
We are not doing it in the right way as OpenWrt.

1. We don't use ``include/u-boot.mk`` from OpenWrt
This means that no flags are being passed to U-boot. Thus we are losing LOCALVERSION [2], which we added to OpenWrt [3]

2. We are compiling Turris Omnia U-boot twice
This leads to increasing build time. Someone can argue that OpenWrt has older version of U-boot, then we are shipping, but we can do it by patch against their version. I was updating U-boot in the past there and testing it.

3. All our U-boot packages are compiled by ``-j1``.
Caused by 1st point. We missed ``PKG_BUILD_PARALLEL``.

4. We are doing weird solutions, which are not necessary and are already handled within OpenWrt build system.
While compiling two configs for one board, it does not do clean (a875a676), thus there can be some leftovers from previous compile run, but if you do it as OpenWrt does it, then it behaves correctly. See: build@b63cf66e

[1] https://oftc.irclog.whitequark.org/openwrt-devel/2022-09-05#31325363;
[2] https://github.com/openwrt/openwrt/blob/master/include/u-boot.mk#L48
[3] https://github.com/openwrt/openwrt/commit/d6aa9d9e071d9f23ed26f5142991bc66aefe20f5