Give more flexibility to define configs that can be interpreted by make, e.g. to
define fallback values of configs like in the example below.

Before this change, the config lines:
 #define CONFIG_SPL_MAX_SIZE	2048
 #define CONFIG_SPL_PAD_TO	CONFIG_SPL_MAX_SIZE
would have been changed in autoconfig.mk into:
 CONFIG_SPL_MAX_SIZE=2048
 CONFIG_SPL_PAD_TO="CONFIG_SPL_MAX_SIZE"

Hence, a make recipe using as an argument to $(OBJCOPY):
 --pad-to=$(CONFIG_SPL_PAD_TO)
would have issued:
 --pad-to="CONFIG_SPL_MAX_SIZE"
which means nothing for $(OBJCOPY) and makes it fail.

Thanks to this change, the config lines above are changed in autoconfig.mk into:
 CONFIG_SPL_MAX_SIZE=2048
 CONFIG_SPL_PAD_TO=$(CONFIG_SPL_MAX_SIZE)

Hence, the make recipe above now issues:
 --pad-to=2048
as expected from the defined config.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Reviewed-by: Tom Rini <trini@ti.com>