ext4: implement exists() for ext4fs

This hooks into the generic "file exists" support added in an earlier
patch, and provides an implementation for the ext4 filesystem.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/include/ext4fs.h b/include/ext4fs.h
index 2429380..aacb147 100644
--- a/include/ext4fs.h
+++ b/include/ext4fs.h
@@ -134,6 +134,7 @@
 int ext4fs_mount(unsigned part_length);
 void ext4fs_close(void);
 int ext4fs_ls(const char *dirname);
+int ext4fs_exists(const char *filename);
 void ext4fs_free_node(struct ext2fs_node *node, struct ext2fs_node *currroot);
 int ext4fs_devread(lbaint_t sector, int byte_offset, int byte_len, char *buf);
 void ext4fs_set_blk_dev(block_dev_desc_t *rbdd, disk_partition_t *info);