sandbox: Makefile changes to build sandbox architecture

At this point U-Boot will build and run on x86 under Linux.

The idea is to define a new architecture called 'sandbox', alongside ARM
and x86. This runs natively on Linux to suit the host machine. All
hardware access is either omitted or emulated.

The purpose of this system is to test the bulk of the non-hardware-specific
U-Boot code. We can mock the SPI flash, GPIOs, UART and keyboard, then test
that U-Boot behaves as we wish.

Signed-off-by: Simon Glass <sjg@chromium.org>