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/Cubietruck_defconfig b/configs/Cubietruck_defconfig
index a4f7b87..fc692ff 100644
--- a/configs/Cubietruck_defconfig
+++ b/configs/Cubietruck_defconfig
@@ -14,6 +14,7 @@
 CONFIG_AHCI=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_SPL_I2C=y
+CONFIG_SYS_PBSIZE=1024
 CONFIG_CMD_DFU=y
 CONFIG_CMD_USB_MASS_STORAGE=y
 CONFIG_SCSI_AHCI=y