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/nokia_rx51_defconfig b/configs/nokia_rx51_defconfig
index 309cf28..a8707b4 100644
--- a/configs/nokia_rx51_defconfig
+++ b/configs/nokia_rx51_defconfig
@@ -28,6 +28,7 @@
 # CONFIG_SYS_DEVICE_NULLDEV is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Nokia RX-51 # "
+CONFIG_SYS_PBSIZE=287
 # CONFIG_CMD_BDI is not set
 CONFIG_CMD_BOOTZ=y
 # CONFIG_BOOTM_NETBSD is not set