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/rcar3_salvator-x_defconfig b/configs/rcar3_salvator-x_defconfig
index 97b3f85..d2233d7 100644
--- a/configs/rcar3_salvator-x_defconfig
+++ b/configs/rcar3_salvator-x_defconfig
@@ -24,6 +24,7 @@
 CONFIG_UPDATE_TFTP=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=2068
 CONFIG_CMD_BOOTZ=y
 CONFIG_SYS_EEPROM_PAGE_WRITE_DELAY_MS=10
 CONFIG_CMD_DFU=y