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/stemmy_defconfig b/configs/stemmy_defconfig
index f49f970..6ae4c39 100644
--- a/configs/stemmy_defconfig
+++ b/configs/stemmy_defconfig
@@ -15,6 +15,7 @@
 CONFIG_BOOTCOMMAND="run fastbootcmd"
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_CONFIG=y
 CONFIG_CMD_LICENSE=y
 CONFIG_CMD_DM=y