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/sifive_unleashed_defconfig b/configs/sifive_unleashed_defconfig
index a174474..1960359 100644
--- a/configs/sifive_unleashed_defconfig
+++ b/configs/sifive_unleashed_defconfig
@@ -25,6 +25,7 @@
 CONFIG_SPL_DM_SPI_FLASH=y
 CONFIG_SPL_DM_RESET=y
 CONFIG_SPL_SPI_LOAD=y
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=276
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SPL_DM_SEQ_ALIAS=y