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/snapper9260_defconfig b/configs/snapper9260_defconfig
index 4c93ba4..5e75102 100644
--- a/configs/snapper9260_defconfig
+++ b/configs/snapper9260_defconfig
@@ -20,6 +20,7 @@
 CONFIG_RESET_PHY_R=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Snapper> "
+CONFIG_SYS_PBSIZE=282
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_FLASH is not set