Convert CONFIG_SYS_CBSIZE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_CBSIZE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/maltael_defconfig b/configs/maltael_defconfig
index 3ae26d2..866f34f 100644
--- a/configs/maltael_defconfig
+++ b/configs/maltael_defconfig
@@ -15,6 +15,7 @@
 CONFIG_MISC_INIT_R=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="maltael # "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=283
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_IDE=y