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_neo_air_defconfig b/configs/nanopi_neo_air_defconfig
index 806d95c..23c1527 100644
--- a/configs/nanopi_neo_air_defconfig
+++ b/configs/nanopi_neo_air_defconfig
@@ -7,5 +7,6 @@
 # CONFIG_VIDEO_DE2 is not set
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_CONSOLE_MUX=y
+CONFIG_SYS_PBSIZE=1024
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_OHCI_HCD=y