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/integratorcp_cm946es_defconfig b/configs/integratorcp_cm946es_defconfig
index 14f0bb7..017aaf7 100644
--- a/configs/integratorcp_cm946es_defconfig
+++ b/configs/integratorcp_cm946es_defconfig
@@ -21,6 +21,7 @@
 # CONFIG_CMDLINE_EDITING is not set
 # CONFIG_AUTO_COMPLETE is not set
 CONFIG_SYS_PROMPT="Integrator-CP # "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=289
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_ARMFLASH=y