From: Erik Andersen <andersen@codepoet.org>

When someone specifies "init=" to select an alternative binary to run
instead of /sbin/init, argv[0] is not set correctly.  This is a problem for
programs such as busybox that multiplex applications based on the value of
argv[0].  For example, even if you specify init=/bin/sh" on the kernel
command line, busybox will still receive "/sbin/init" as argv[0] and will
therefore run init rather than /bin/sh...