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/stmark2_defconfig b/configs/stmark2_defconfig
index 75369d2..64ac661 100644
--- a/configs/stmark2_defconfig
+++ b/configs/stmark2_defconfig
@@ -18,6 +18,7 @@
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMDLINE_EDITING is not set
 CONFIG_SYS_PROMPT="stmark2 $ "
+CONFIG_SYS_PBSIZE=283
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_EXPORTENV is not set