Support copying of kernel tagged list (atags) in the decompressor.

This is needed to hardboot kexec a kernel with a new tags list (including a
new kernel command line), since the new atags would otherwise be lost with
the limited kernel memory mapping.  Without this patch, a hardboot-kexec'd
kernel uses the atags provided by the bootloader.
Support hard booting to a kexec kernel.

See KEXEC_HARDBOOT config option help for details.

Change-Id: I18a787d4e4b16cab047463f5e9fea9ecf55609c5