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/qemu-x86_64_defconfig b/configs/qemu-x86_64_defconfig
index d1ad802..0e66303 100644
--- a/configs/qemu-x86_64_defconfig
+++ b/configs/qemu-x86_64_defconfig
@@ -23,6 +23,7 @@
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="root=/dev/sdb3 init=/sbin/init rootwait ro"
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
+CONFIG_LAST_STAGE_INIT=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_CPU_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y