aspeed: Introduce a 'boot-emmc' machine option

aspeed: Introduce a 'boot-emmc' machine option

The default behavior of some Aspeed machines is to boot from the eMMC
device, like the rainier-bmc. Others like ast2600-evb could also boot
from eMMC if the HW strapping boot-from-eMMC bit was set. Add a
property to set or unset this bit. This is useful to test boot images.

For now, only activate this property on the ast2600-evb and rainier-bmc
machines for which eMMC images are available or can be built.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Tested-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>