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/T4240RDB_SDCARD_defconfig b/configs/T4240RDB_SDCARD_defconfig
index 43354c0..6d27081 100644
--- a/configs/T4240RDB_SDCARD_defconfig
+++ b/configs/T4240RDB_SDCARD_defconfig
@@ -36,6 +36,7 @@
 CONFIG_SPL_I2C=y
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_GREPENV=y
 CONFIG_CMD_DM=y