mmc: Move MMC boot code into its own file

Rather than having an #ifdef in the main mmc.c file, control this feature
from the Makefile by moving the code into its own file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mmc/Makefile b/drivers/mmc/Makefile
index 3da4817..b44a12e 100644
--- a/drivers/mmc/Makefile
+++ b/drivers/mmc/Makefile
@@ -25,6 +25,9 @@
 obj-$(CONFIG_FTSDC010) += ftsdc010_mci.o
 obj-$(CONFIG_FTSDC021) += ftsdc021_sdhci.o
 obj-$(CONFIG_GENERIC_MMC) += mmc.o
+ifdef CONFIG_SUPPORT_EMMC_BOOT
+obj-$(CONFIG_GENERIC_MMC) += mmc_boot.o
+endif
 obj-$(CONFIG_GENERIC_ATMEL_MCI) += gen_atmel_mci.o
 obj-$(CONFIG_KONA_SDHCI) += kona_sdhci.o
 obj-$(CONFIG_MMC_SPI) += mmc_spi.o