package/elfutils: set program-prefix to default ("eu-")

package/elfutils: set program-prefix to default ("eu-")

Elfutils program names collide with binutils' binaries. By default
applications provided by elfutils are prefixed with "eu-", but in
Buildroot that setting is overridden by pkg-autotools.
The option in pkg-autotools rules was added to avoid including a target
triple in some packages as a prefix, so restore elfutils default
behaviour by adding a "eu-" program-prefix.

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: add comment in the .mk file explaining why we have a custom
program prefix, as suggested by Arnout.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>