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/sagem_f@st1704_ram_defconfig b/configs/sagem_f@st1704_ram_defconfig
index fd246b1..48dd55c 100644
--- a/configs/sagem_f@st1704_ram_defconfig
+++ b/configs/sagem_f@st1704_ram_defconfig
@@ -19,6 +19,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="F@ST1704 # "
 CONFIG_SYS_MAXARGS=24
+CONFIG_SYS_PBSIZE=540
 CONFIG_CMD_CPU=y
 CONFIG_CMD_LICENSE=y
 # CONFIG_CMD_BOOTD is not set