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/nsim_hs38_defconfig b/configs/nsim_hs38_defconfig
index 8cbad86..357d61f 100644
--- a/configs/nsim_hs38_defconfig
+++ b/configs/nsim_hs38_defconfig
@@ -16,6 +16,7 @@
 CONFIG_BOOTARGS="console=ttyS0,115200n8"
 CONFIG_BOARD_EARLY_INIT_R=y
 CONFIG_SYS_PROMPT="nsim# "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=279
 CONFIG_CMD_DM=y
 # CONFIG_CMD_SETEXPR is not set