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/ls1021atsn_sdcard_defconfig b/configs/ls1021atsn_sdcard_defconfig
index e6fdec2..82b38a8 100644
--- a/configs/ls1021atsn_sdcard_defconfig
+++ b/configs/ls1021atsn_sdcard_defconfig
@@ -40,6 +40,7 @@
 CONFIG_SPL_I2C=y
 CONFIG_SPL_MPC8XXX_INIT_DDR=y
 CONFIG_SPL_WATCHDOG=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_GREPENV=y
 CONFIG_SYS_I2C_EEPROM_ADDR_LEN=2
 CONFIG_CMD_MEMINFO=y