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/kontron_pitx_imx8m_defconfig b/configs/kontron_pitx_imx8m_defconfig
index 07a07bd..cfc7635 100644
--- a/configs/kontron_pitx_imx8m_defconfig
+++ b/configs/kontron_pitx_imx8m_defconfig
@@ -35,6 +35,7 @@
 CONFIG_SPL_I2C=y
 CONFIG_SPL_POWER=y
 CONFIG_SPL_WATCHDOG=y
+CONFIG_SYS_PBSIZE=276
 # CONFIG_BOOTM_NETBSD is not set
 # CONFIG_CMD_EXPORTENV is not set
 # CONFIG_CMD_IMPORTENV is not set