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/sam9x60_curiosity_mmc_defconfig b/configs/sam9x60_curiosity_mmc_defconfig
index dd884bf..d0064de 100644
--- a/configs/sam9x60_curiosity_mmc_defconfig
+++ b/configs/sam9x60_curiosity_mmc_defconfig
@@ -27,6 +27,7 @@
 CONFIG_MISC_INIT_R=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
+CONFIG_SYS_PBSIZE=281
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_CLK=y
 CONFIG_CMD_DM=y