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/at91sam9m10g45ek_mmc_defconfig b/configs/at91sam9m10g45ek_mmc_defconfig
index 03e6d3a..c0ca368 100644
--- a/configs/at91sam9m10g45ek_mmc_defconfig
+++ b/configs/at91sam9m10g45ek_mmc_defconfig
@@ -25,6 +25,7 @@
 CONFIG_RESET_PHY_R=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
+CONFIG_SYS_PBSIZE=281
 # CONFIG_CMD_BDI is not set
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set