dosfstools: fix static linking with libiconv

Fixes http://autobuild.buildroot.net/results/246/246c067f1e9def748498f0c6fa6988c036e1d109/



Pass the libraries to be linked in LDLIBS instead of LDFLAGS so the end up
after the object files on the linker command line.

While we are at it, use TARGET_CONFIGURE_OPTS instead of explictly passing
CC/CFLAGS/LDFLAGS.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>