Makefile: move the common makefile line to public area

Move the common makefile line shared by the SPL and non-SPL to the public area,
so that we can avoid excessive SPL symbols. Some of them will be used by the
SPL later.

This patch is on top of the patch "common/Makefile: Add new symbol
CONFIG_SPL_ENV_SUPPORT for environment in SPL".

Signed-off-by: Ying Zhang <b40530@freescale.com>
Acked-by: Tom Rini <trini@ti.com>
Acked-by: Tom Rini <trini@ti.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>