Comment 0 for bug 1659618

[IMPACT]
 * kexec-tools in Xenial (16.04) currently does not support ARM64 architecture.
 * Backport support for ARM64 arch from upstream https://github.com/horms/kexec-tools
 * Majority of the patches are contained in kexec/arch/arm64/ except for one patch that impacts
   purgatory and common device tree routines.

[TEST CASE]
 * I built kexec-tools for ARM64 and tested it on HW using the following testcase:
   $ sudo kexec -l /boot/vmlinuz-<version>-generic --initrd=/boot/initrd.img-<version>-generic
     --command-line="root=UUID=<uuid> ro vt.handoff=7"
   $ sudo kexec -e
 * I was able to kexec the new kernel successfully.
 * [ 6702.357899] kexec_core: Starting new kernel
   [ 0.000000] Booting Linux on physical CPU 0x200
   [ 0.000000] Linux version <version>-generic (buildd@bos01-arm64-008) (gcc version 5.4.0
   20160609 (Ubuntu/Linaro 5.4.0-6ubuntu1~16.04.4) )
   [ 0.000000] Boot CPU: AArch64 Processor [510f8000]

[REGRESSION POTENTIAL]
 * Since patches are confined to arm[64] there is a low overall risk of regression.

[OTHER INFO]
 * You can find a Xenial kexec-tools package built for AMD64, i386 and ARM64 in my PPA
    https://launchpad.net/~manjo/+archive/ubuntu/devtools/
 * This package is built using the Xenial source package for kexec-tools with ARM64 enablement
   patches applied.
 * Please pull the changes from my PPA package and integrate into Ubuntu Xenial kexec-tools after
   review.