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/sama5d3xek_mmc_defconfig b/configs/sama5d3xek_mmc_defconfig
index 654b5fd..7428305 100644
--- a/configs/sama5d3xek_mmc_defconfig
+++ b/configs/sama5d3xek_mmc_defconfig
@@ -35,6 +35,7 @@
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set