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/imx6ul_geam_mmc_defconfig b/configs/imx6ul_geam_mmc_defconfig
index a113dda..a84547f 100644
--- a/configs/imx6ul_geam_mmc_defconfig
+++ b/configs/imx6ul_geam_mmc_defconfig
@@ -32,6 +32,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="geam6ul> "
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=538
 CONFIG_CRC32_VERIFY=y
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_GPIO=y