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/hsdk_4xd_defconfig b/configs/hsdk_4xd_defconfig
index b043982..8a45517 100644
--- a/configs/hsdk_4xd_defconfig
+++ b/configs/hsdk_4xd_defconfig
@@ -19,6 +19,7 @@
 CONFIG_BOARD_LATE_INIT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="hsdk-4xd# "
+CONFIG_SYS_PBSIZE=2075
 CONFIG_CMD_ENV_FLAGS=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y