ARM: tegra: select between Seaboard/Ventana at compile time

ARM: tegra: select between Seaboard/Ventana at compile time

Seaboard and Ventana are very similar boards, and so share the seaboard.c
board file. The one difference needed so far is detected at run-time by
calling machine_is_ventana(). This bloats the Ventana build with code
that is never used. Switch to detecting Ventana at compile time to remove
bloat. This shaves ~5K off the SPL size on Ventana, and makes the SPL fit
within the max size.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>