Add and use plat_crash_console_flush() API

This API makes sure that all the characters sent to the crash console
are output before returning from it.

Porting guide updated.

Change-Id: I1785f970a40f6aacfbe592b6a911b1f249bb2735
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/include/plat/common/platform.h b/include/plat/common/platform.h
index 73bb643..e8dfb99 100644
--- a/include/plat/common/platform.h
+++ b/include/plat/common/platform.h
@@ -90,6 +90,7 @@
 void plat_report_exception(unsigned int exception_type);
 int plat_crash_console_init(void);
 int plat_crash_console_putc(int c);
+int plat_crash_console_flush(void);
 void plat_error_handler(int err) __dead2;
 void plat_panic_handler(void) __dead2;