dtc: Refactor Makefiles (v2)

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> 
> Here's a corrected version.
> 
> dtc: Refactor Makefiles
> 
> This patch makes a number of Makefile cleanups and improvements:
> 	- We use more generic rules to invoke flex and bison, which is
> useful for some of the other changes.
> 	- We use the name dtc-lexer.lex.c for the flex output, instead
> of the default lex.yy.c.  That means less potential for confusion if
> dtc is embedded into other projects (e.g. the kernel).
> 	- We separate out a Makefile.dtc designed for embedding into
> other projects, analagous to Makefile.libfdt.
> 	- Makefile.libfdt is cleaned up to be more useful based on
> some actual trial runs of embedding libfdt in the kernel bootwrapper.
> 	- Versioning related rules and variables are collected into
> one place in the Makefile.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

Thanks,
jdl