initrd: Detach from the world and build on the fly

Why
---
Currently, the files for initrd image creation are built and installed at
/usr/share/initrd by "buildworld" and "installworld", respectivel

initrd: Detach from the world and build on the fly

Why
---
Currently, the files for initrd image creation are built and installed at
/usr/share/initrd by "buildworld" and "installworld", respectively, and then
mkinitrd(8) simply packs them to create the initrd.img.gz.

For normal users, the shipped /usr/share/initrd and mkinitrd(8) don't make
much sense, since they don't need to modify the initrd contents and create
a modified initrd image.

From a developer's perspective, the required steps (buildworld, installworld
and mkinitrd) to create and test a new initrd image can be annoying.

How
---
Detach the build and installation of the initrd contents from the world.
Build the (rescue/initrd) tools on-the-fly when creating the initrd image.

As per ftigeot's suggestion, these statically linked tools can be useful when
dealing with a broken system, so install them under /rescue to be more
intuitive and easier to use (similar to FreeBSD and maybe other BSDs).

What
----
* Move "share/initrd" to be top-level, and detach it from world.

+ Update the patch in bsd.crunchgen.mk accordingly.
+ Add Makefile.inc to simplify subdir's makefiles.
+ Rewrite the Makefile with targets:
- rescue: build and install the rescue tools into /rescue.
- initrd: further packs the etc and rescue staffs to create the initrd
image.

* Move "sbin/mkinitrd/mkinitrd.sh" under the new "initrd" directory and adapt
it to work with the "initrd" make target. Remove the other parts of
mkinitrd(8).

* Export the above "rescue" and "initrd" targets to be top-level.

* Update nrelease framework to use the new "make initrd".

* Update build.7 man page accordingly.

* Print the message about updating the initrd image and 3rd-party packages
after "make upgrade" instead of "make installworld".

* Document this change in UPDATING.

* Other small updates and style cleanups.

TODO
----
* Add initrd.7 man page
* Crunch more useful tools
* Crunch bin/sbin/etc. into a bundle to save space (??)

Reviewed-by: swildner
Thanks-to: dillon, swildner, ftigeot

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