Some of the patches we are preparing for porting ATF from NetBSD
refer to '/usr/libexec' several times. Instead of repeating the
path all over the place, add ${LIBEXECDIR} to match ${LIBDATADIR}
and reduce the redundancy of the relevant makefiles.
Submitted by: Garrett Cooper <yanegomi@gmail.com>
Approved by: ru
MFC after: 1 month
61e05fce5e