autoconfig.mk: Make it possible to define configs from other configs

autoconfig.mk: Make it possible to define configs from other configs

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>