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_nor_lpuart_defconfig b/configs/ls1021atwr_nor_lpuart_defconfig
index aa6b619..802225b 100644
--- a/configs/ls1021atwr_nor_lpuart_defconfig
+++ b/configs/ls1021atwr_nor_lpuart_defconfig
@@ -31,6 +31,7 @@
 CONFIG_ARCH_MISC_INIT=y
 CONFIG_MISC_INIT_R=y
 CONFIG_ID_EEPROM=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_GREPENV=y
 CONFIG_SYS_EEPROM_PAGE_WRITE_DELAY_MS=5