koch --nim:pathto/nim boot and koch boot --hint:cc:off now work

[timotheecour]

• koch boot --hint:cc:off now works (previously silently ignored that option)
  hintCC (the CC: passaux.nim lines etc) are very verbose so it's important that user should be able to disable these.
• koch --nim:pathto/nim boot now works (previsouly silently ignored that option)
• code cleanup to avoid duplications in affected code

# will honor --hint:cc:off or --hint:processing:off or any other flag in user configs
./koch boot -d:release --skipUserCfg:off
# other example that now works:
./koch --nim:pathto/nim boot -d:release -d:nimUseLinenoise --hint:cc:off

• TODO for future PR
  Now, in combination with Fix #9405 - cfg and nims run in sync; correctly honor cmdline --hint:conf:on/off ; correctly show Conf hints in order #13488 and this PR, we can have a perfect workaround for isolate the build process from external configuration files #13411 (isolate the build process from external configuration files) if we allow build_all.sh to forward arguments to the koch build commands:

sh build_all.sh --skipUserCfg:off --hint:cc:off # for example

[Araq]

Now, in combination with #13488 and this PR, we can have a perfect workaround for #13411 (isolate the build process from external configuration files) if we allow build_all.sh to forward arguments to the koch build commands:

Yeah but the point of build_all.sh is that you don't have to know about these details.

[timotheecour]

thanks for merging