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/ls1021atwr_sdcard_ifc_defconfig b/configs/ls1021atwr_sdcard_ifc_defconfig
index 8bab45b..813bf7f 100644
--- a/configs/ls1021atwr_sdcard_ifc_defconfig
+++ b/configs/ls1021atwr_sdcard_ifc_defconfig
@@ -48,6 +48,7 @@
 CONFIG_SPL_I2C=y
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
 CONFIG_SPL_WATCHDOG=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_GREPENV=y
 CONFIG_SYS_EEPROM_PAGE_WRITE_DELAY_MS=5