ARM: read kernel size from zImage

Read the new extension data which tells the boot agent about the
requirements for booting the kernel image, such as how much RAM
will be consumed by the kernel through decompression and booting.
This is necessary to control the placement of the DTB and
compressed RAM disk to avoid these objects being corrupted.

Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk@armlinux.org.uk>
Signed-off-by: Simon Horman <horms@verge.net.au>