ubldr: Force 'usefdt' variable to 1 for powerpc

The fdt is gated, on powerpc, with a 'usefdt' environment variable.  Force
enable it in ubldr, so that the fdt is passed through the metadata.

stand/uboot/common/main.c

@@ -497,6 +497,9 @@ main(int argc, char **argv)
 do_interact:
 	setenv("LINES", "24", 1);		/* optional */
 	setenv("prompt", "loader>", 1);
+#ifdef __powerpc__
+	setenv("usefdt", "1", 1);
+#endif
 
 	archsw.arch_loadaddr = uboot_loadaddr;
 	archsw.arch_getdev = uboot_getdev;