Add a flag indicating when the serial console is ready

For sandbox we have a fallback console which is used very early in
U-Boot, before serial drivers are available. Rather than try to guess
when to switch to the real console, add a flag so we can be sure. This
makes sure that sandbox can always output a panic() message, for example,
and avoids silent failure (which is very annoying in sandbox).

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/asm-generic/global_data.h b/include/asm-generic/global_data.h
index edde9d7..74df210 100644
--- a/include/asm-generic/global_data.h
+++ b/include/asm-generic/global_data.h
@@ -106,5 +106,6 @@
 #define GD_FLG_LOGINIT		0x00020	/* Log Buffer has been initialized */
 #define GD_FLG_DISABLE_CONSOLE	0x00040	/* Disable console (in & out)	   */
 #define GD_FLG_ENV_READY	0x00080	/* Env. imported into hash table   */
+#define GD_FLG_SERIAL_READY	0x00100	/* Pre-reloc serial console ready  */
 
 #endif /* __ASM_GENERIC_GBL_DATA_H */