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/integratorap_cm720t_defconfig b/configs/integratorap_cm720t_defconfig
index 7f5e470..72da7b8 100644
--- a/configs/integratorap_cm720t_defconfig
+++ b/configs/integratorap_cm720t_defconfig
@@ -17,6 +17,7 @@
 # CONFIG_CMDLINE_EDITING is not set
 # CONFIG_AUTO_COMPLETE is not set
 CONFIG_SYS_PROMPT="Integrator-AP # "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=289
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_ARMFLASH=y