sdhc_boot: Introduce CONFIG_FSL_FIXED_MMC_LOCATION option

Since commit 97039ab98 (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>
diff --git a/include/configs/P1010RDB.h b/include/configs/P1010RDB.h
index af4609f..5edd1aa 100644
--- a/include/configs/P1010RDB.h
+++ b/include/configs/P1010RDB.h
@@ -609,6 +609,7 @@
 #if defined(CONFIG_SYS_RAMBOOT)
 #if defined(CONFIG_RAMBOOT_SDCARD)
 #define CONFIG_ENV_IS_IN_MMC
+#define CONFIG_FSL_FIXED_MMC_LOCATION
 #define CONFIG_SYS_MMC_ENV_DEV		0
 #define CONFIG_ENV_SIZE			0x2000
 #elif defined(CONFIG_RAMBOOT_SPIFLASH)