util-linux: explicitly use /usr/{bin, sbin, lib}, with merged usr

util-linux: explicitly use /usr/{bin, sbin, lib}, with merged usr

Prevent the installation from attempting to move shared libraries from
${usrlib_execdir} to ${libdir} if both paths are the same, which leads
to error messages like this:

  mv: '$(TARGET_DIR)/usr/lib/libfoo.so.1' and '$(TARGET_DIR)/lib/libfoo.so.1' are the same file

That error is not fatal but let's avoid possible future problems.

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>