ARM: tegra: remove Seaboard board files

The Seaboard device tree supports all the features that the Seaboard
board files support. Hence, there's no need to keep the board files
around any more; all users should convert to device tree.

MACH_KAEN and MACH_WARIO are also removed. While tegra-seaboard.dts
doesn't support those explicitly, it would be trivial to create device
trees for those boards if anyone cares.

The Seaboard device tree is now compiled if Tegra2 support is enabled,
rather than when Seaboard support is enabled.

Signed-off-by: Stephen Warren <swarren@nvidia.com>