tegra: rename board directory for Colibri T20 ...

For computer modules the naming standard is to have
a single board directory named after the module and
have all the baseboard support beneath it.

Also change the CONFIG name, as we may want to build
all the baseboards at once.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

tegra: switch to multi image ...

To keep things clean I removed all support for the old way to build
images. There is now a single tegra_v7 defconfig which builds both
supported Tegra boards as images.
The new image generation also paves the way for integration of the
tegra-cbootimage tool to produce directly flashable images.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>