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/sama5d27_giantboard_defconfig b/configs/sama5d27_giantboard_defconfig
index 5d1bad8..610f85a 100644
--- a/configs/sama5d27_giantboard_defconfig
+++ b/configs/sama5d27_giantboard_defconfig
@@ -37,6 +37,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_MISC_INIT_R=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_I2C=y