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/linkit-smart-7688_defconfig b/configs/linkit-smart-7688_defconfig
index 8ffe8a5..b137edd 100644
--- a/configs/linkit-smart-7688_defconfig
+++ b/configs/linkit-smart-7688_defconfig
@@ -32,6 +32,7 @@
 CONFIG_SPL_NOR_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_LICENSE=y
 # CONFIG_CMD_ELF is not set