elfutils: make it possible to only install the libraries

Add and use a --{enable,disable}-progs configuration option to
selectively enable or disable the elfutils programs. Generally, on an
embedded system, the libraries are more useful than the programs, and
being able to not build the programs will make it easier to build the
elfutils libraries on uClibc.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>