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/vexpress_aemv8a_semi_defconfig b/configs/vexpress_aemv8a_semi_defconfig
index 7e4a5cc..561474c 100644
--- a/configs/vexpress_aemv8a_semi_defconfig
+++ b/configs/vexpress_aemv8a_semi_defconfig
@@ -15,6 +15,7 @@
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SYS_PROMPT="VExpress64# "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=541
 CONFIG_CMD_ABOOTIMG=y
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_ARMFLASH=y