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/blanche_defconfig b/configs/blanche_defconfig
index b21c5fd..01e9ea3 100644
--- a/configs/blanche_defconfig
+++ b/configs/blanche_defconfig
@@ -19,6 +19,7 @@
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=3
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=256
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set