dtc: Improve the make install target

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> This patch makes various improvements to dtc's make install target:
> 	- libfdt is also installed.  Specifically, libfdt.a and the
> two export relevant header files, fdt.h and libfdt.h are installed.
> 	- ftdump is no longer installed.  It was only ever a
> development debugging tool and may well go away at some point.
> 	- In keeping with normal conventions, there is now a PREFIX
> variable, allowing control of where things are installed (in /usr,
> /usr/local, /opt, etc.).
> 	- By default, installed into the user's home directory,
> instead of /usr.  This is friendlier for self-installers, package
> builders can easily override PREFIX to restore the old behaviour.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

Thanks,
jdl