Disable VGA text console for AVR32 architecture

This patch disables the VGA text console for AVR32 architecture since
it does not provide the vga.h include file.

AVR32 users should use framebuffer console instead if they need a
console on an attached display.

Signed-off-by: Hans-Christian Egtvedt <hcegtvedt@atmel.com>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>