define UART_CONSOLE_INDEX using Kconfig

Use Kconfig to define this variable and remove its
previous definitions from board.h. This will simplify
creation of different variants from a single platform.

Change-Id: I0aaa5aa81dedf096c6d8c1ea2d509c71817336d3
Signed-off-by: Anas Nashif <anas.nashif@intel.com>