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/axm_defconfig b/configs/axm_defconfig
index 607e1f7..d17cdd2 100644
--- a/configs/axm_defconfig
+++ b/configs/axm_defconfig
@@ -45,6 +45,7 @@
 CONFIG_SPL_DM_SPI_FLASH=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=281
 # CONFIG_CMD_BDI is not set
 CONFIG_CMD_BOOTZ=y