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/odroid-xu3_defconfig b/configs/odroid-xu3_defconfig
index a4c9c79..17ef8ad 100644
--- a/configs/odroid-xu3_defconfig
+++ b/configs/odroid-xu3_defconfig
@@ -25,6 +25,7 @@
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_MISC_INIT_R=y
 CONFIG_SYS_PROMPT="ODROID-XU3 # "
+CONFIG_SYS_PBSIZE=1024
 CONFIG_CMD_THOR_DOWNLOAD=y
 CONFIG_CMD_DFU=y
 CONFIG_CMD_GPIO=y