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/dockstar_defconfig b/configs/dockstar_defconfig
index d591c4a..eb67978 100644
--- a/configs/dockstar_defconfig
+++ b/configs/dockstar_defconfig
@@ -22,6 +22,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="DockStar> "
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=1051
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_NAND=y
 CONFIG_CMD_USB=y