common/Makefile: Add new symbol CONFIG_SPL_ENV_SUPPORT for environment in SPL

There will need the environment in SPL for reasons other than network
support (in particular, hwconfig contains info for how to set up DDR).

Add a new symbol CONFIG_SPL_ENV_SUPPORT to replace CONFIG_SPL_NET_SUPPORT
for environment in common/Makefile.

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