hw/arm/boot: Avoid placing the initrd on top of the kernel

hw/arm/boot: Avoid placing the initrd on top of the kernel

We currently put the initrd at the smaller of:
 * 128MB into RAM
 * halfway into the RAM
(with the dtb following it).

However for large kernels this might mean that the kernel
overlaps the initrd. For some kinds of kernel (self-decompressing
32-bit kernels, and ELF images with a BSS section at the end)
we don't know the exact size, but even there we have a
minimum size. Put the initrd at least further into RAM than
that. For image formats that can give us an exact kernel size, this
will mean that we definitely avoid overlaying kernel and initrd.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Message-id: 20190516144733.32399-4-peter.maydell@linaro.org