Add a switch_root utility (like kconfig's utils/run_init.c, although not

actuall using any of that code).  This is needed because pivot_root doesn't
work right under initramfs.  (See the menuconfig help.)

include/applets.h

@@ -625,6 +625,9 @@
 #ifdef CONFIG_SWAPONOFF
 	APPLET(swapon, swap_on_off_main, _BB_DIR_SBIN, _BB_SUID_NEVER)
 #endif
+#ifdef CONFIG_SWITCH_ROOT
+	APPLET(switch_root, switch_root_main, _BB_DIR_SBIN, _BB_SUID_NEVER)
+#endif
 #ifdef CONFIG_SYNC
 	APPLET(sync, sync_main, _BB_DIR_BIN, _BB_SUID_NEVER)
 #endif

include/usage.h

@@ -2735,6 +2735,12 @@
 	"Options:\n" \
 	"\t-a\tStart swapping on all swap devices"
 
+#define switch_root_trivial_usage \
+	"NEW_ROOT NEW_INIT [ARGUMENTS_TO_INIT]"
+#define switch_root_full_usage \
+	"Use from PID 1 under initramfs to free initramfs, chroot to NEW_ROOT,\n" \
+	"and exec NEW_INIT.\n"
+
 #define sync_trivial_usage \
 	""
 #define sync_full_usage \