env_mmc: Allow board code to override the environment address

On some boards the environment may not be located at a fixed address in
the MMC/SDHC card.  This allows those boards to implement their own
means to report what address the environment is located at.

Signed-off-by: Mingkai Hu <Mingkai.hu@freescale.com>
Signed-off-by: Jerry Huang <Chang-Ming.Huang@freescale.com>
Signed-off-by: Zhao Chenhui <b35336@freescale.com>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>