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/sniper_defconfig b/configs/sniper_defconfig
index 1491cdf..483b371 100644
--- a/configs/sniper_defconfig
+++ b/configs/sniper_defconfig
@@ -17,6 +17,7 @@
 # CONFIG_SPL_FS_EXT4 is not set
 # CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_SYS_PROMPT="sniper # "
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=538
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y