boards: x86: ehl_crb: Add board variant for Slim Bootloader

When using Slim Bootloader the UART configuration isn't quite the same
as with the UEFI BIOS. In particular, UART2 is hidden in PCIe and is
instead accessible using a fixed MMIO address. Interrupts are also not
supported for this UART currently.

The simplest way to create builds against this special BIOS/bootloader
setup seems to be to create a new board variant/definition which lets
us provide a custom device tree overlay as well a dedicated Kconfig
default configuration.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>