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/nanopi_m1_plus_defconfig b/configs/nanopi_m1_plus_defconfig
index 37b7817..5ce046c 100644
--- a/configs/nanopi_m1_plus_defconfig
+++ b/configs/nanopi_m1_plus_defconfig
@@ -8,6 +8,7 @@
 CONFIG_MMC0_CD_PIN="PH13"
 CONFIG_MMC_SUNXI_SLOT_EXTRA=2
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SYS_PBSIZE=1024
 CONFIG_SUN8I_EMAC=y
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_OHCI_HCD=y