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/mt7629_rfb_defconfig b/configs/mt7629_rfb_defconfig
index 87e5737..80e5308 100644
--- a/configs/mt7629_rfb_defconfig
+++ b/configs/mt7629_rfb_defconfig
@@ -29,6 +29,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
 CONFIG_SYS_MAXARGS=8
+CONFIG_SYS_PBSIZE=1049
 CONFIG_CMD_BOOTMENU=y
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_XIMG is not set