bitbake.conf/cross.bbclass: Add ability to dynamically change library location

bitbake.conf/cross.bbclass: Add ability to dynamically change library location

Add the ability to dynamically change the library directory from "lib" to
other values. This allows the tune files to specifiy altnerative ABIs
which can be dynamically enabled by the multilib BBCLASSEXTEND code.

A variety of approaches have been attempted with this, the immediate
expansions in cross.bbclass being problematic as they are they are expanded
before the bbclass extend event hander runs. This approach ensures the
${baselib} variable is retained in the expressions resolving that complication.

Derived from some ideas from Mark Hatle.

(From OE-Core rev: aeea22da699b276a97ca1a17e3c53176c9afd9de)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>