x86: Allow use of serial soon after relocation

At present on x86 machines with use cache-as-RAM, the memory goes away just
before board_init_r() is called. This means that serial drivers are
no-longer unavailable, until initr_dm() it called, etc.

Any attempt to use printf() within this period will cause a hang.

To fix this, mark the serial devices as 'unavailable' when it is no-longer
available. Bring it back when serial_initialize() is called. This means that
the debug UART will be used instead for this period.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

common/board_f.c

@@ -1095,6 +1095,13 @@ void board_init_f_r(void)
 	if (initcall_run_list(init_sequence_f_r))
 		hang();
 
+	/*
+	 * The pre-relocation drivers may be using memory that has now gone
+	 * away. Mark serial as unavailable - this will fall back to the debug
+	 * UART if available.
+	 */
+	gd->flags &= ~GD_FLG_SERIAL_READY;
+
 	/*
 	 * U-Boot has been copied into SDRAM, the BSS has been cleared etc.
 	 * Transfer execution from Flash to RAM by calculating the address

drivers/serial/serial-uclass.c

@@ -115,7 +115,7 @@ int serial_init(void)
 /* Called after relocation */
 void serial_initialize(void)
 {
-	serial_find_console_or_panic();
+	serial_init();
 }
 
 static void _serial_putc(struct udevice *dev, char ch)