initramfs-module-kexecboot: Initramfs module to kexec to kernel in rootfs.

initramfs-module-kexecboot: Initramfs module to kexec to kernel in rootfs.
* Instead of switch_root'ing to it, which was the only working mode previously.
So, now, if there's /boot/zImage in rootfs, it will be kexec'ed too. Otherwise,
switch_root will happen.
* Caveat: there should be initramfs for new (kexec'ed to) kernel too, too handle
its loading of rootfs as selected by interactive bootloader. This can be very
same kernel as for bootloader, with bundled initramfs-bootmenu-image (which
has special check - if root is explicitly specified on command line, then skip
interactive selection); or, as a kind of optimization, that could be initramfs-image
(no interactive part). Finally, initramfs (any of there two or other) can be
a standalone file in rootfs, /boot/initramfs.bin.