tools: add a generic config for native tools building

The motivation for this is to allow distributions to distribute all
possible tools in a generic way, avoiding the need of specific tools
building for each machine.

Especially on OpenEmbedded / Yocto Project ecosystem, it is very
common each BSP to end providing their specific tools when they need
to generate images for some SoC (e.g MX23 / MX28 in meta-freescale
case).

Using this, we can package the tools doing:

$: make tools-only_defconfig
$: make tools-only

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
[trini: Add MAINTAINERS entry for myself, add to .travis.yml, make
U-Boot itself buildable to not trip up other frameworks]
Signed-off-by: Tom Rini <trini@konsulko.com>

.travis.yml

@@ -336,6 +336,10 @@ matrix:
     - name: "Check for configs without MAINTAINERS entry"
       script:
         - if [ `./tools/genboardscfg.py -f 2>&1 | wc -l` -ne 0 ]; then exit 1; fi
+    # Ensure host tools build
+    - name: "Build tools-only"
+      script:
+        - make tools-only_config tools-only -j$(nproc)
 
     # test/py
     - name: "test/py sandbox"