treewide: Migrate CONFIG_LAST_STAGE_INIT to Kconfig

Migrate the CONFIG_LAST_STAGE_INIT option to Kconfig.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/configs/strider_con_defconfig b/configs/strider_con_defconfig
index f024e47..b860250 100644
--- a/configs/strider_con_defconfig
+++ b/configs/strider_con_defconfig
@@ -14,6 +14,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_BOARD_EARLY_INIT_R=y
+CONFIG_LAST_STAGE_INIT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_STOP_STR=" "