fix(app-crypt/sbsigntool) fix cross-build for sbsigntool

As the documentation isn't being built, don't try to install it.  Fix
this by just not ever going into the doc/ subdirectory.  I'm sure
there's a "proper" fix for this with the HELP2MAN stuff, but it really
doesn't matter, so this work-around is good enough for now.
Merge pull request #189 from gregkh/sbsigntool-fork

fix(app-crypt/sbsigntool) fix cross-build for sbsigntool