Implement kexec-hardboot

"Allows hard booting (i.e., with a full hardware reboot) to a kernel
previously loaded in memory by kexec. This works around the problem of
soft-booted kernel hangs due to improper device shutdown and/or
reinitialization."
More info in /arch/arm/Kconfig.

Original author: Mike Kasick <mike@kasick.org>

Vojtech Bocek <vbocek@gmail.com>:
I've ported it to grouper it based on Asus TF201 patches
ported by Jens Andersen <jens.andersen@gmail.com>.

I've moved atags copying from guest to the host kernel, which means there
is no need to patch the guest kernel, assuming the --mem-min in kexec call
is within the first 256MB of System RAM, otherwise it will take a long time
to load. I've also fixed /proc/atags entry, which would give the kexec-tools
userspace binary only the first 1024 bytes of atags,
see arch/arm/kernel/atags.c for more details.