python-pyelftools is not packaged for RHEL/CentOS with
the exception of RHEL7 EPEL.
Add command to install it with pip.
Fixes: f0f93a7adfee ("buildtools: use Python pmdinfogen")
Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>