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/kzm9g_defconfig b/configs/kzm9g_defconfig
index 45881b5..4477966 100644
--- a/configs/kzm9g_defconfig
+++ b/configs/kzm9g_defconfig
@@ -18,6 +18,7 @@
 # CONFIG_CMDLINE_EDITING is not set
 # CONFIG_AUTO_COMPLETE is not set
 CONFIG_SYS_PROMPT="KZM-A9-GT# "
+CONFIG_SYS_PBSIZE=256
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_I2C=y