Wait a maximum of 300 seconds for network send/recv operations

[bcran]

Summary

The reason for this change is that currently, a send/recv appears to take many hours to time out.
This is suboptimal in the bootloader because it means for example that NFS will take hours to fail before allowing subsequent access methods such as gzip to be tried.

Setting MAXWAIT to 300 seconds (5 minutes) still allows slow connections of 1Mb to be used to download a 30MB kernel file.
For example 1Mb hubs may still be used in some places, or some people may be booting over an Internet connection (e.g.SpeedTest reports a couple of sites in Antarctica having ~1Mb connections - https://www.speedtest.net/insights/blog/exploring-internet-antarctica-2017/).

[bcran]

The reason for this change is that currently, a send/recv appears to take many hours to time out. This is suboptimal in the bootloader because it means for example that NFS will take hours to fail before allowing subsequent access methods such as gzip to be tried.

[bcran]

Increase MAXWAIT to 300, giving enough time to download a 30MB kernel file over a 1Mb connection.

[imp]

Sure, any number here is better than 0. 60 or 300. I don't care.