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/colibri-imx6ull_defconfig b/configs/colibri-imx6ull_defconfig
index 30445e8..750b03b 100644
--- a/configs/colibri-imx6ull_defconfig
+++ b/configs/colibri-imx6ull_defconfig
@@ -24,6 +24,7 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_SYS_PROMPT="Colibri iMX6ULL # "
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=547
 # CONFIG_BOOTM_PLAN9 is not set
 # CONFIG_BOOTM_RTEMS is not set
 # CONFIG_CMD_ELF is not set