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/pxe.c b/cmd/pxe.c
index 982e2b1..37b8dea 100644
--- a/cmd/pxe.c
+++ b/cmd/pxe.c
@@ -27,7 +27,7 @@
 };
 
 static int do_get_tftp(struct pxe_context *ctx, const char *file_path,
-		       char *file_addr, ulong *sizep)
+		       char *file_addr, enum bootflow_img_t type, ulong *sizep)
 {
 	char *tftp_argv[] = {"tftp", NULL, NULL, NULL};
 	int ret;