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/boston64r2el_defconfig b/configs/boston64r2el_defconfig
index f3f70f9..326c757 100644
--- a/configs/boston64r2el_defconfig
+++ b/configs/boston64r2el_defconfig
@@ -21,6 +21,7 @@
 CONFIG_FIT_BEST_MATCH=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_PROMPT="boston # "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=282
 # CONFIG_CMD_ELF is not set
 CONFIG_CMD_GREPENV=y