bitbake: set default libexecdir to $prefix/libexec
The use of $libdir/$BPN as libexecdir is contrary to all other mainstream
distributions (which either use $prefix/libexec or $libdir), and the GNU Coding
Standards[1] which suggests $prefix/libexec and notes that any package-specific
nesting should be done by the package itself. Finally, having libexecdir change
between recipes makes it very difficult for different recipes to invoke binaries
that have been installed into libexecdir. The File System Hierarchy[2] now
recognises the use of $prefix/libexec/, giving distributions the choice of
$prefix/lib or $prefix/libexec without breaking FHS.
Change bitbake.conf to use $prefix/libexec for libexecdir, so that the binaries
are separated from the libraries. This should avoid complications with multilib
configurations.
