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/dragonboard410c_defconfig b/configs/dragonboard410c_defconfig
index 28ce34c..32c1417 100644
--- a/configs/dragonboard410c_defconfig
+++ b/configs/dragonboard410c_defconfig
@@ -20,6 +20,7 @@
 CONFIG_MISC_INIT_R=y
 CONFIG_SYS_PROMPT="dragonboard410c => "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=548
 # CONFIG_CMD_IMI is not set
 CONFIG_CMD_MD5SUM=y