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>