Fixes ticket:297. Refactors how usrp2/firmware is built. Now
AC_CONFIG_SUBDIRS is always called, and there's a AM_CONDITIONAL in
usrp2/firmware that decides whether anything really gets built or not.
I think this will fix the "double cross" issue (cross-compiling the
bulk of this for cell while cross-compiling the usrp2 firmware at the
same time), though it's not tested yet.
git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@9695 221aa14e-8319-0410-a670-987f0aec2ac5