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/sam9x60ek_qspiflash_defconfig b/configs/sam9x60ek_qspiflash_defconfig
index 45a332d..13d08ff 100644
--- a/configs/sam9x60ek_qspiflash_defconfig
+++ b/configs/sam9x60ek_qspiflash_defconfig
@@ -28,6 +28,7 @@
 CONFIG_MISC_INIT_R=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
+CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=281
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_CLK=y