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/poplar_defconfig b/configs/poplar_defconfig
index 8479b62..e5b7f5b 100644
--- a/configs/poplar_defconfig
+++ b/configs/poplar_defconfig
@@ -13,6 +13,7 @@
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SYS_PROMPT="poplar# "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=537
 CONFIG_CMD_MMC=y
 CONFIG_CMD_USB=y
 # CONFIG_ISO_PARTITION is not set