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/hikey960_defconfig b/configs/hikey960_defconfig
index f4b28b9..7921bea 100644
--- a/configs/hikey960_defconfig
+++ b/configs/hikey960_defconfig
@@ -17,6 +17,7 @@
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_MISC_INIT_R=y
 CONFIG_SYS_PROMPT="U-Boot => "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=283
 CONFIG_CMD_MD5SUM=y
 CONFIG_CMD_MEMINFO=y