Some people boot images with the entry point in the middle of the blob
(like Linux with the head code in discardable .init.text), and there is no
no real requirement that the entry point be right after the mkimage header
when doing XIP, so let people specify whatever they want.  If they do need
an entry right after the header, then they still can do that with normal
-e behavior.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>