spl: env_common.c: make CONFIG_SPL_BUILD contain function env_import

The functionality env_import will be used in the SPL. They
had been excluded by ifndef CONFIG_SPL_BUILD. Now, put it
into the SPL.

Signed-off-by: Ying Zhang <b40530@freescale.com>
Acked-by: Tom Rini <trini@ti.com>
Acked-by: York Sun <yorksun@freescale.com>