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/huawei_hg556a_ram_defconfig b/configs/huawei_hg556a_ram_defconfig
index 32e6abc..5de0fe9 100644
--- a/configs/huawei_hg556a_ram_defconfig
+++ b/configs/huawei_hg556a_ram_defconfig
@@ -20,6 +20,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="HG556a # "
 CONFIG_SYS_MAXARGS=24
+CONFIG_SYS_PBSIZE=538
 CONFIG_CMD_CPU=y
 CONFIG_CMD_LICENSE=y
 # CONFIG_CMD_BOOTD is not set