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_SECURE_BOOT_defconfig b/configs/ls1021atwr_nor_SECURE_BOOT_defconfig
index 628e1d4..1ddd433 100644
--- a/configs/ls1021atwr_nor_SECURE_BOOT_defconfig
+++ b/configs/ls1021atwr_nor_SECURE_BOOT_defconfig
@@ -30,6 +30,7 @@
 CONFIG_SYS_CONSOLE_IS_IN_ENV=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