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/bcm7260_defconfig b/configs/bcm7260_defconfig
index 024cad1..2804220 100644
--- a/configs/bcm7260_defconfig
+++ b/configs/bcm7260_defconfig
@@ -20,6 +20,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot>"
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=536
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_GPT=y