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/mx6sllevk_plugin_defconfig b/configs/mx6sllevk_plugin_defconfig
index 975e2ed..bd44298 100644
--- a/configs/mx6sllevk_plugin_defconfig
+++ b/configs/mx6sllevk_plugin_defconfig
@@ -19,6 +19,7 @@
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_GPIO=y