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/imx8mn_beacon_defconfig b/configs/imx8mn_beacon_defconfig
index 9b7da66..a3e46bf 100644
--- a/configs/imx8mn_beacon_defconfig
+++ b/configs/imx8mn_beacon_defconfig
@@ -41,6 +41,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="u-boot=> "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=2074
 # CONFIG_BOOTM_NETBSD is not set
 # CONFIG_CMD_EXPORTENV is not set
 # CONFIG_CMD_IMPORTENV is not set