hw/arm_boot.c: move initrd load address up to accommodate large kernels

hw/arm_boot.c: move initrd load address up to accommodate large kernels

Newer kernels are large enough that they can overlap the address
where qemu places the initrd. Move the initrd up so that there is
enough space for the kernel again.

Unfortunately it's not possible to automatically determine the
size of the kernel if it is compressed, so this is the best we
can do.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>