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/poplar_defconfig b/configs/poplar_defconfig
index e5b7f5b..f0c2040 100644
--- a/configs/poplar_defconfig
+++ b/configs/poplar_defconfig
@@ -13,6 +13,7 @@
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SYS_PROMPT="poplar# "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=537
 CONFIG_CMD_MMC=y
 CONFIG_CMD_USB=y