[Request] Add flag to prevent deletion of build files

[TheCrypt0]

This would be great especially when running local server builds: https://community.mongoose-os.com/t/optimize-private-server-builds/2498/5?u=thecrypt0

At the moment it seems like it always gets executed: build_remote.go#L61

[TheCrypt0]

@rojer Any update on this? I would love to submit a PR with the update but I've never really worked in go and don't have the machine configured to work on this codebase.

Not really a PR but in pseudocode:

Step 1: Create the --keep-build-dir (KeepBuildDir) flag in flags.go
Step 2: Check if *flags.KeepBuildDir is true and if so, skip the os.RemoveAll(buildDir) build_remote.go#L61
Step 3: Profit!

Let me know if this simple change can be integrated 😄

[plinss]

I submitted two PRs for this, I think I prefer the approach in #77