console: Implement pre-console buffer

Allow redirection of console output prior to console initialisation to a
temporary buffer.

To enable this functionality, the board (or arch) must define:
 - CONFIG_PRE_CONSOLE_BUFFER - Enable pre-console buffer
 - CONFIG_PRE_CON_BUF_ADDR - Base address of pre-console buffer
 - CONFIG_PRE_CON_BUF_SZ - Size of pre-console buffer (in bytes)

The pre-console buffer will buffer the last CONFIG_PRE_CON_BUF_SZ bytes
Any earlier characters are silently dropped.
diff --git a/arch/blackfin/include/asm/global_data.h b/arch/blackfin/include/asm/global_data.h
index eba5e93..f7aa711 100644
--- a/arch/blackfin/include/asm/global_data.h
+++ b/arch/blackfin/include/asm/global_data.h
@@ -45,6 +45,9 @@
 	unsigned long board_type;
 	unsigned long baudrate;
 	unsigned long have_console;	/* serial_init() was called */
+#ifdef CONFIG_PRE_CONSOLE_BUFFER
+	unsigned long	precon_buf_idx;	/* Pre-Console buffer index */
+#endif
 	phys_size_t ram_size;		/* RAM size */
 	unsigned long env_addr;	/* Address  of Environment struct */
 	unsigned long env_valid;	/* Checksum of Environment valid? */