Convert CONFIG_CMD_JFFS2 to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_JFFS2

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/omap3_evm_defconfig b/configs/omap3_evm_defconfig
index 1b61d1d..227f1d1 100644
--- a/configs/omap3_evm_defconfig
+++ b/configs/omap3_evm_defconfig
@@ -21,6 +21,7 @@
 CONFIG_CMD_PING=y
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_JFFS2=y
 CONFIG_EFI_PARTITION=y
 # CONFIG_PARTITION_UUIDS is not set
 # CONFIG_SPL_PARTITION_UUIDS is not set