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/verdin-imx8mm_defconfig b/configs/verdin-imx8mm_defconfig
index 1f77b30..4489213 100644
--- a/configs/verdin-imx8mm_defconfig
+++ b/configs/verdin-imx8mm_defconfig
@@ -40,6 +40,7 @@
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_PROMPT="Verdin iMX8MM # "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=2081
 # CONFIG_BOOTM_NETBSD is not set
 CONFIG_CMD_ASKENV=y
 # CONFIG_CMD_EXPORTENV is not set