rootfs-handoff: Milestone B - add early userspace /init

rootfs-handoff: Milestone B - add early userspace /init

- Created user/cmds/initrd_init/initrd_init.c as early /init program
- Created user/cmds/initrd_init/Makefile
- Added initrd_init to USER_CMD_NAMES in main Makefile
- Changed default init path from /sbin/init to /init in cmdline.c
- Early /init checks if /newroot is mounted by kernel
- If /newroot is mounted, executes /sbin/init (basic handoff)
- If /newroot is not mounted, falls back to /sbin/init directly
- Milestone C will add pivot_root and mount moving functionality

Test Results:
- Smoke test: 131/131 PASS
- Battery test: 157/157 PASS
- Analyzer: PASS
- Zero regressions