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/stv0991_defconfig b/configs/stv0991_defconfig
index 27d1f40..d03d984 100644
--- a/configs/stv0991_defconfig
+++ b/configs/stv0991_defconfig
@@ -23,6 +23,7 @@
 CONFIG_BOARD_EARLY_INIT_F=y
 # CONFIG_AUTO_COMPLETE is not set
 CONFIG_SYS_PROMPT="STV0991> "
+CONFIG_SYS_PBSIZE=1050
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_SPI=y
 # CONFIG_CMD_SETEXPR is not set