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/mx6slevk_spl_defconfig b/configs/mx6slevk_spl_defconfig
index 03f0bb4..828922c 100644
--- a/configs/mx6slevk_spl_defconfig
+++ b/configs/mx6slevk_spl_defconfig
@@ -30,6 +30,7 @@
 CONFIG_SPL_WATCHDOG=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y