examples/udhcp/simple.script: fix IPv6 support when using udhcpc

examples/udhcp/simple.script: fix IPv6 support when using udhcpc

The udhcpc script calls ip addr flush .. which flushes addresses
of any address family, including IPv6. However, busybox udhcpc is
IPv4 only and should not influence IPv6 addressing. Hence use ip
addr flush with family constraint.

The script particularly broke IPv6 SLAAC: Typically when udhcpc
calls the script the kernel already assigned the IPv6 link-local
address. The flush removes the link-local IPv6 address again and
prohibits proper IPv6 operation such as SLAAC since neighbor
discovery protocol relies on IPv6 link-local addressing.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
[Taken from https://git.openembedded.org/openembedded-core/commit/meta/recipes-core/busybox/files/simple.script?id=b77541dbb2f442e51842f9d24c8745a6df2d1478]
Signed-off-by: Quentin Schulz <quentin.schulz@streamunlimited.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>