xen/arm: build as zImage

xen/arm: build as zImage

The zImage format is extremely simple: it only consists of a magic
number and 2 addresses in a specific position (see
http://www.simtec.co.uk/products/SWLINUX/files/booting_article.html#d0e309).

Some bootloaders expect a zImage; considering that it doesn't cost us
much to build Xen compatible with the format, make it so.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- switch from 7*nop + nop to just 8*nop ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>