toolchain: make IPv6 mandatory for external toolchains

Remove BR2_INET_IPV6 select for predefined external toolchains.

Remove the (non)IPv6 option prompt since it's now mandatory.

And force the toolchain check now that internal uclibc is always built
with IPv6 support and external non-IPv6 toolchains are disallowed.

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