x86: Fix the mystery of printch() during 64-bit boot

At present in arch_setup_gd() it calls printch(' ') at the end which
has been a mystery for a long time as without such call the 64-bit
U-Boot just does not boot at all.

In fact this is due to the bug that board_init_f() was called with
boot_flags not being set. Hence whatever value being there in the
rdi register becomes the boot_flags if without such magic call.
With a printch(' ') call the rdi register is initialized as 0x20
and this value seems to be sane enough for the whole boot process.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

arch/x86/cpu/start64.S

@@ -20,6 +20,7 @@ _start:
 
 	call	board_init_f_init_reserve
 
+	xor	%rdi, %rdi
 	call	board_init_f
 	call	board_init_f_r
 

arch/x86/cpu/x86_64/cpu.c

@@ -19,24 +19,6 @@ struct global_data *global_data_ptr = (struct global_data *)~0;
 void arch_setup_gd(gd_t *new_gd)
 {
 	global_data_ptr = new_gd;
-
-	/*
-	 * TODO(sjg@chromium.org): For some reason U-Boot does not boot
-	 * without this line. It fails to start up U-Boot proper and instead
-	 * restarts SPL. Need to figure out why:
-	 *
-	 * U-Boot SPL 2017.01
-	 *
-	 * U-Boot SPL 2017.01
-	 * CPU:   Intel(R) Core(TM) i5-3427U CPU @ 1.80GHz
-	 * Trying to boot from SPIJumping to 64-bit U-Boot: Note many
-	 * features are missing
-	 *
-	 * U-Boot SPL 2017.01
-	 */
-#ifdef CONFIG_DEBUG_UART
-	printch(' ');
-#endif
 }
 
 int cpu_has_64bit(void)