x86: Implement panic output for coreboot

panic_puts() can be called in early boot to display a message. It might
help with early debugging.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>

arch/x86/include/asm/u-boot-x86.h

@@ -33,6 +33,7 @@ void init_gd(gd_t *id, u64 *gdt_addr);
 void setup_gdt(gd_t *id, u64 *gdt_addr);
 int init_cache(void);
 int cleanup_before_linux(void);
+void panic_puts(const char *str);
 
 /* cpu/.../timer.c */
 void timer_isr(void *);