fix(check_deps): Depend exclusively on ld.so.conf for lib list

The list in ld.so.conf needs to be complete so don't bother with
including other hard-coded paths or the path to ld-linux-x86-64.so.2.
fix(install_toolchain): Exclude libc locale/info/etc from build root.

These excludes are used in images and I want to use the same checking
code as images do so the build root needs to behave the same way.

This is temporary as I'll switch to installing libc via emerge soon.
fix(test_build_root): Switch to using test_image_content

Start using the more complete test_image_content test for the build root
and add the test to the end of build_packages.
fix(test_image_content): Add test for /usr file conflicts.
fix(test_image_content): Clean out some stray tab chars
Merge pull request #137 from marineam/check-pkgs

Improvements for image validation, add /usr conflict test