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_spiflash_defconfig b/configs/sama5d3xek_spiflash_defconfig
index 4c6fc3d..20ea57f 100644
--- a/configs/sama5d3xek_spiflash_defconfig
+++ b/configs/sama5d3xek_spiflash_defconfig
@@ -38,6 +38,7 @@
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x10000
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set