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/pg_wcom_seli8_defconfig b/configs/pg_wcom_seli8_defconfig
index 7a9515b..56e922a 100644
--- a/configs/pg_wcom_seli8_defconfig
+++ b/configs/pg_wcom_seli8_defconfig
@@ -42,6 +42,7 @@
 CONFIG_LAST_STAGE_INIT=y
 CONFIG_MISC_INIT_R=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_CBSIZE=512
 CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_GREPENV=y