Convert CONFIG_MTD_CONCAT to Kconfig

This converts the following to Kconfig:
  CONFIG_MTD_CONCAT

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/configs/etamin_defconfig b/configs/etamin_defconfig
index ba7a066..34b9fff 100644
--- a/configs/etamin_defconfig
+++ b/configs/etamin_defconfig
@@ -87,6 +87,7 @@
 # CONFIG_SPL_DM_MMC is not set
 CONFIG_MMC_OMAP_HS=y
 CONFIG_MTD=y
+CONFIG_MTD_CONCAT=y
 CONFIG_MTD_RAW_NAND=y
 CONFIG_NAND_OMAP_ECCSCHEME_BCH16_CODE_HW=y
 CONFIG_SYS_NAND_BLOCK_SIZE=0x80000