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/nsa310s_defconfig b/configs/nsa310s_defconfig
index aad5d1f..978570c 100644
--- a/configs/nsa310s_defconfig
+++ b/configs/nsa310s_defconfig
@@ -22,6 +22,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="NSA310s> "
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=1050
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_NAND=y