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/bcm968380gerg_ram_defconfig b/configs/bcm968380gerg_ram_defconfig
index d861c14..d35a2ba 100644
--- a/configs/bcm968380gerg_ram_defconfig
+++ b/configs/bcm968380gerg_ram_defconfig
@@ -19,6 +19,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="bcm968380gerg # "
 CONFIG_SYS_MAXARGS=24
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=545
 CONFIG_CMD_CPU=y
 CONFIG_CMD_LICENSE=y