toolchain-external: make sure (e)glibc isn't chosen when BR2_PREFER_STATIC_LIB=y

(e)glibc doesn't support a fully statically linked userspace. Even a
basic program such as Busybox fails to do authentication due to glibc
loading some libraries dynamically. Therefore, we disable the
possibility of using a (e)glibc toolchain when
BR2_PREFER_STATIC_LIB=y.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>