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/at91sam9n12ek_mmc_defconfig b/configs/at91sam9n12ek_mmc_defconfig
index 9cb1ea2..9ab9eff 100644
--- a/configs/at91sam9n12ek_mmc_defconfig
+++ b/configs/at91sam9n12ek_mmc_defconfig
@@ -20,6 +20,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
+CONFIG_SYS_PBSIZE=281
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y