Build all packages in out/build

This makes it easier to remove all builds by removing a single directory
(without removing the sysroot)
Fix environment for compiling user-space programs

Fix the environment such as the compilation of user-space programs
finally succeeds. This includes:

- Set various PKG_CONFIG_* variables to that pkg-config knows where to
  look (and *not* to look) for its library configuration files
- Use the system pkg-config instead of the Linaro toolchain's, which is
  old and does not handle PKG_CONFIG_SYSROOT_DIR correctly. We hope that
  the local pkg-config is new enough.
- Fix CCFLAGS and CXXFLAGS so the compiler uses the rootfs as its
  sysroot and knows where to look at
- Add a --with-sysroot option to the call to autogen.sh so that
  libtool knows it needs to prefix library paths in .la files with it

While we are at it, we also get the right number of processors to pass
to make -j.
Add a list of Ubuntu packages to install on the L4T image