tools: arm: improve placement of initial modules.

tools: arm: improve placement of initial modules.

314c9815e2f5 "tools: implement initial ramdisk support for ARM." broke starting
guests with <= 128 MB ram by placing the boot modules (dtb and initrd)
immediately after the kernel in this case, running the risk of them being
overwritten. Instead place the modules at the end of RAM, as the hypervisor
does for dom0.

The hypervisor also falls back to placing things before the kernel as a last
resort before failing, so add that here too.

Tested with the Debian installer initrd and guests of 96MB, 128MB, 256MB and
1GB. All work, also tested with 64MB but the installer doesn't run with so
little RAM (but our placement of the initrd is correct).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>