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/iot2050_defconfig b/configs/iot2050_defconfig
index 089b98c..01ef500 100644
--- a/configs/iot2050_defconfig
+++ b/configs/iot2050_defconfig
@@ -46,6 +46,7 @@
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x280000
 CONFIG_SYS_PROMPT="IOT2050> "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=1050
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_DFU=y
 CONFIG_CMD_GPT=y