LocalLinuxTarget: fix executables folder location

Since:
   cafc0a4b target: do not create shutil in package directory
we generate 'shutils' in /tmp, which is also the executables_path
used by default by a LocalLinuxTarget.

This ultimately results into a:

   self.install(shutils_ofile)
      ...
         shutil.copy(source, dest)

raising an exception since source == dest.

Let's fix this by setting /tmp/devlib-target as a default base path for
all devlib deployed stuff into a localhost target.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>