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/peach-pi_defconfig b/configs/peach-pi_defconfig
index 7c45872..2a2a0e0 100644
--- a/configs/peach-pi_defconfig
+++ b/configs/peach-pi_defconfig
@@ -23,6 +23,7 @@
 CONFIG_SILENT_CONSOLE=y
 # CONFIG_SPL_FRAMEWORK is not set
 CONFIG_SYS_PROMPT="Peach-Pi # "
+CONFIG_SYS_PBSIZE=1024
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_MMC=y