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>