Convert CONFIG_CONSOLE_SCROLL_LINES to Kconfig

This converts the following to Kconfig:
   CONFIG_CONSOLE_SCROLL_LINES

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/som-db5800-som-6867_defconfig b/configs/som-db5800-som-6867_defconfig
index 715311f..4b01663 100644
--- a/configs/som-db5800-som-6867_defconfig
+++ b/configs/som-db5800-som-6867_defconfig
@@ -60,4 +60,5 @@
 CONFIG_FRAMEBUFFER_SET_VESA_MODE=y
 CONFIG_FRAMEBUFFER_VESA_MODE_11A=y
 CONFIG_CFB_CONSOLE=y
+CONFIG_CONSOLE_SCROLL_LINES=5
 CONFIG_USE_PRIVATE_LIBGCC=y