bitbake.conf: change libexecdir to ${libdir}/${BPN}

bitbake.conf: change libexecdir to ${libdir}/${BPN}

In order to be more compliant with the Filesystem Hierarchs
Standard (FHS), this change removes the /usr/libexec default
in favor of ${libdir}/${BPN} (which is typically /usr/lib).

http://www.pathname.com/fhs/pub/fhs-2.3.html

This also address the native and STAGING variations

[YOCTO #2915]

(From OE-Core rev: 68c31b095a1cb20bd297df596024fc568614f5e8)

(From OE-Core rev: 406bd38b4232f9f399ef5ffe0b4fac72ed605a23)

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>