util-linux: add -lcurses to LIBS when static linking

When linking statically with -lreadline we also need to specify
-lcurses. We were already passing LIBS=-intl to fix gettext related
issues, so we refactor how LIBS is passed so that several values can be
passed.

It is worth mentioning that:

 1. Passing LIBS= at configure time is not sufficient, because the
    configure script does LIBS="" at its end. So in order for the LIBS
    value to also be effective during the build, it needs to be passed
    in MAKE_OPTS as well.

 2. LIBS=-lintl was only passed at build time (it was not needed for any
    of the configure tests). However, LIBS=-lncurses is needed both at
    configure time (for configure tests) and at build time. Therefore,
    the new variable UTIL_LINUX_LIBS gets passed as the LIBS value at
    both configure time and build time.

Fixes:
  http://autobuild.buildroot.net/results/70e0a273e699c73c6b2ab2a19c8f7ebb1ddee6cde
  http://autobuild.buildroot.net/results/6a7faf8adbffd9437c0117b9f0e89684c24dad23



Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>