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/stm32f769-disco_defconfig b/configs/stm32f769-disco_defconfig
index 89fac36..180112a 100644
--- a/configs/stm32f769-disco_defconfig
+++ b/configs/stm32f769-disco_defconfig
@@ -18,6 +18,7 @@
 CONFIG_BOOTARGS="console=ttyS0,115200 earlyprintk consoleblank=0 ignore_loglevel"
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SYS_PROMPT="U-Boot > "
+CONFIG_SYS_PBSIZE=1050
 CONFIG_CMD_GPT=y
 # CONFIG_RANDOM_UUID is not set
 CONFIG_CMD_MMC=y