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/mx7ulp_com_defconfig b/configs/mx7ulp_com_defconfig
index b9154a9..ac062c3 100644
--- a/configs/mx7ulp_com_defconfig
+++ b/configs/mx7ulp_com_defconfig
@@ -17,6 +17,7 @@
 CONFIG_DEFAULT_FDT_FILE="imx7ulp-com"
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y