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/uDPU_defconfig b/configs/uDPU_defconfig
index ed13a2b..8097141 100644
--- a/configs/uDPU_defconfig
+++ b/configs/uDPU_defconfig
@@ -25,6 +25,7 @@
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SYS_PROMPT="uDPU>> "
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=1048
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_XIMG is not set