boot: Update extlinux pxe_getfile_func() to include type

Add a file-type parameter to this function and update all users. Add a
proper comment to the function which we are here.

This will allow tracking of the file types loaded by the extlinux
bootmeth.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/sysboot.c b/cmd/sysboot.c
index 8a06078..93d4a40 100644
--- a/cmd/sysboot.c
+++ b/cmd/sysboot.c
@@ -23,7 +23,8 @@
 };
 
 static int sysboot_read_file(struct pxe_context *ctx, const char *file_path,
-			     char *file_addr, ulong *sizep)
+			     char *file_addr, enum bootflow_img_t type,
+			     ulong *sizep)
 {
 	struct sysboot_info *info = ctx->userdata;
 	loff_t len_read;
@@ -110,7 +111,8 @@
 		return CMD_RET_FAILURE;
 	}
 
-	if (get_pxe_file(&ctx, filename, pxefile_addr_r) < 0) {
+	if (get_pxe_file(&ctx, filename, pxefile_addr_r)
+	    < 0) {
 		printf("Error reading config file\n");
 		pxe_destroy_ctx(&ctx);
 		return 1;