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/snapper9g20_defconfig b/configs/snapper9g20_defconfig
index 34f563c..25c2e19 100644
--- a/configs/snapper9g20_defconfig
+++ b/configs/snapper9g20_defconfig
@@ -19,6 +19,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_RESET_PHY_R=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_FLASH is not set