ustr: new package

[Thomas:
 - Fix typo in patch description.
 - Remove trailing whitespace in Config.in.
 - Fix Config.in comment to match what's specified in the Buildroot
   manual.
 - Adjust .mk logic to use BR2_STATIC_LIBS vs. BR2_SHARED_LIBS instead
   of BR2_PREFER_STATIC_LIB, and therefore support the installation of
   only the shared library.
 - Remove the libustr-debug library from the target and staging
   directories, it's the same as libustr, except that it's compiled
   with debugging symbols.
 - Always install only the shared variant of the library for
   host-ustr.]

Signed-off-by: Clayton Shotwell <clshotwe@rockwellcollins.com>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>