sdhc_boot: Introduce CONFIG_FSL_FIXED_MMC_LOCATION option

Since commit 97039ab9 (env_mmc: Allow board code to override the environment address)
mmc_get_env_addr is a weak-aliased function in common/env_mmc.c

The mmc_get_env_addr implementation that exists at
board/freescale/common/sdhc_boot.c is meant to be used only for PowerPC boards,
but currently it is being used for all platforms that have CONFIG_ENV_IS_IN_MMC defined.

Introduce CONFIG_FSL_FIXED_MMC_LOCATION so that the boards that need to use
the mmc_get_env_addr version from board/freescale/common/sdhc_boot.c could activate
this config option on their board file.

This fixes the retrieval of CONFIG_ENV_OFFSET on non-PowerPC boards.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Stefano Babic <sbabic@denx.de>