Convert CONFIG_SYS_PBSIZE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_PBSIZE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/gwventana_nand_defconfig b/configs/gwventana_nand_defconfig
index 16e94bc..e022527 100644
--- a/configs/gwventana_nand_defconfig
+++ b/configs/gwventana_nand_defconfig
@@ -52,6 +52,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Ventana > "
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=539
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_SPL_NAND_OFS=0x1100000
 CONFIG_CMD_SPL_WRITE_SIZE=0x20000