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/kmopti2_defconfig b/configs/kmopti2_defconfig
index 92b2945..cd34d5d 100644
--- a/configs/kmopti2_defconfig
+++ b/configs/kmopti2_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SYS_EXTRA_OPTIONS="KMOPTI2"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_BOARD_EARLY_INIT_R=y
+CONFIG_LAST_STAGE_INIT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Hit <SPACE> key to stop autoboot in %2ds\n"