riscv: qemu: Enable PRE_CONSOLE_BUFFER

By default the video console only outputs messages after it's ready.
Messages before that won't show on the video console, but U-Boot has
an option to buffer the console messages before it's ready.

Enable this support, and carefully select an address for the buffer.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/board/emulation/qemu-riscv/Kconfig b/board/emulation/qemu-riscv/Kconfig
index 976c350..7220c55 100644
--- a/board/emulation/qemu-riscv/Kconfig
+++ b/board/emulation/qemu-riscv/Kconfig
@@ -25,6 +25,10 @@
 	hex
 	default 0x80100000
 
+config PRE_CON_BUF_ADDR
+	hex
+	default 0x81000000
+
 config BOARD_SPECIFIC_OPTIONS # dummy
 	def_bool y
 	select GENERIC_RISCV
@@ -71,5 +75,6 @@
 	imply VIDEO
 	imply VIDEO_BOCHS
 	imply SYS_WHITE_ON_BLACK
+	imply PRE_CONSOLE_BUFFER
 
 endif