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/axs103_defconfig b/configs/axs103_defconfig
index d86fb9c..bb2f783 100644
--- a/configs/axs103_defconfig
+++ b/configs/axs103_defconfig
@@ -17,6 +17,7 @@
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="AXS# "
+CONFIG_SYS_PBSIZE=278
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SPI=y