uclibc: don't install tools to staging when static

Don't try to install ld* tools to staging when it's a static build,
there's no point and it'll be broken once we build a purely-static
uclibc.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>