add more package building / debugging documentation

[eyberg]

I routinely trip over a few things when building packages:

1. not having the loader specified - easy to fix by showing

 exec_debug("n is -> %s\n", n);

on L211 in exec_elf

2. not having libc libs present but necessary for things like clock_monotic (librt) or dns (libnss_*) - debug would be

ops run -d -c config.json /usr/lib/jvm/java-11-openjdk-amd64/bin/java &>/tmp/o

then grepping for any libs probed but not loaded

there's a few other things but these are some common mistakes

[eyberg]

https://github.com/nanovms/ops/issues/1365