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/T2080RDB_NAND_defconfig b/configs/T2080RDB_NAND_defconfig
index 1c73bfb..c6df1fc 100644
--- a/configs/T2080RDB_NAND_defconfig
+++ b/configs/T2080RDB_NAND_defconfig
@@ -38,6 +38,7 @@
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_IMLS=y
 CONFIG_SYS_I2C_EEPROM_ADDR_LEN=2
 CONFIG_CMD_MEMTEST=y