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/microblaze-generic_defconfig b/configs/microblaze-generic_defconfig
index 6823d41..31d2c48 100644
--- a/configs/microblaze-generic_defconfig
+++ b/configs/microblaze-generic_defconfig
@@ -31,6 +31,7 @@
 CONFIG_SYS_OS_BASE=0x2c060000
 CONFIG_SYS_PROMPT="U-Boot-mONStR> "
 CONFIG_SYS_MAXARGS=15
+CONFIG_SYS_PBSIZE=544
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_SPL=y
 CONFIG_CMD_ASKENV=y