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/Hummingbird_A31_defconfig b/configs/Hummingbird_A31_defconfig
index 24e8b5b..20144b2 100644
--- a/configs/Hummingbird_A31_defconfig
+++ b/configs/Hummingbird_A31_defconfig
@@ -9,6 +9,7 @@
 CONFIG_VIDEO_VGA_VIA_LCD=y
 CONFIG_VIDEO_VGA_EXTERNAL_DAC_EN="PH25"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SYS_PBSIZE=1024
 CONFIG_PHY_REALTEK=y
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_RGMII=y