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/T2080QDS_NAND_defconfig b/configs/T2080QDS_NAND_defconfig
index f5fc3e1..ec54a71 100644
--- a/configs/T2080QDS_NAND_defconfig
+++ b/configs/T2080QDS_NAND_defconfig
@@ -40,6 +40,7 @@
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_GREPENV=y
 CONFIG_CMD_I2C=y