Pass --static rather than -static in LDFLAGS when BR2_PREFER_STATIC_LIB

The following changes LDFLAGS from -static to --static if building
with BR2_PREFER_STATIC_LIB so that various components actually build
statically.

Libtool interpret -static as linking statically with libraries that will not
be installed to the libdir; you have to pass it -all-static to force static
linking.  Or, pass --static, which libtool passes on blindly to gcc.  gcc
and (GNU) ld both interpret --static the same as -static (although this
isn't documented).

Signed-off-by: Andy Kennedy <andy.kennedy@adtran.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>