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 flo, it is based of my grouper port, which is based of
  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.

  Other than that, memory-reservation code for the hardboot page and
  some assembler to do the watchdog reset on MSM chip are new for this device.

Signed-off-by: Vojtech Bocek <vbocek@gmail.com>
kexec: use mem_text_write_kernel_word to set reboot_code_buffer args

* In order to avoid protection faults (writes to read-only
  kernel memory) when CONFIG_STRICT_MEMORY_RWX is enabled.

Signed-off-by: Vojtech Bocek <vbocek@gmail.com>
Enable kexec-hardboot on datflo_defconfig