New board: SolidRun ClearFog Base

My goal was to rely on upstreamed features as much as possible, which
means that some bits are only half-baked for now:

- Due to the DTS restructuring in upstream kernel, we require 4.11+. The
latest LTS or CIP kernels do not know about the -Base model.

- Linux has no generic support for SFP cages (yet). It seems that this
has hit the net-next tree in August 2017, but there's been no release
(it's probably targettting 4.14). Also, the merge only included the
required infrastructure; the mvneta driver conversion is not included.
Patches which finalize this exist in Russel King's tree and also in
random vendor trees.

- There's no access to the SPI flash in these versions of
uboot/linux/dts.

Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
[Arnout:
 - rename to solidrun_clearfog_defconfig;
 - specify kernel headers version (default is now 4.15);
 - remove ext2 fs size override, the 4 extra MB are not needed;
 - U-Boot needs dtc and openssl;
 - add comments to defconfig;
 - update .gitlab-ci.yml;
 - mention in readme.txt that SFP support is missing;
 - add Jan to DEVELOPERS.
]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

board/solidrun/clearfog/linux.fragment

@@ -0,0 +1 @@
+CONFIG_SENSORS_MCP3021=y