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/rzg2_beacon_defconfig b/configs/rzg2_beacon_defconfig
index 608be08..930e3e6 100644
--- a/configs/rzg2_beacon_defconfig
+++ b/configs/rzg2_beacon_defconfig
@@ -21,6 +21,7 @@
 # CONFIG_BOARD_EARLY_INIT_F is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=2068
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y