M1_M3/M6_MX: Wait for net devices to come up

Based on https://github.com/Pivosgroup/buildroot-linux commits 77bc498c48d83e9c6094b769fc0ea6b9d41f2a6a & a083da1c0a7bf153e8e43bf38f2a41efdafb5f63

Signed-off-by: dh-harald <github@criminal.hu>