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/ls1021aqds_nor_defconfig b/configs/ls1021aqds_nor_defconfig
index d5b85db..6461bbe 100644
--- a/configs/ls1021aqds_nor_defconfig
+++ b/configs/ls1021aqds_nor_defconfig
@@ -32,6 +32,7 @@
 CONFIG_MISC_INIT_R=y
 CONFIG_ID_EEPROM=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_GREPENV=y