Auto-disable (deep) caching of core builds if needed

[cmaglie]

When build-path and cache-path are on different partitions it will not be possible (on windows) to reference them with a relative path. This commit allows this by automatically disabling the aggressive core caching.

Previously an error was emitted and the build interrupted.

Should fix #464

[matthijskooijman]

Do we need to refer to them with a relative path? I guess absolute paths add command length, so that is not ideal, but maybe that is ok for just the core? Or are there other problems with absolute paths?

[facchinm]

@matthijskooijman It was just about commandline length, so using the absolute path should solve the issue if coreBuildCachePath.RelTo(ctx.BuildPath) fails

[cmaglie]

The problem is that platform.txt uses (example taken from samd core):

 recipe.c.combine.pattern="{compiler.path}{compiler.c.elf.cmd}"  "-L{build.path}" ...
   -o "{build.path}/{build.project_name}.elf" {object_files} ...
   -lm "{build.path}/{archive_file}" -Wl,--end-group

so we need "{build.path}/{archive_file}" with archive_file relative to build.path

BTW on other parts of platform.txt there is also:

# archive_file_path is needed for backwards compatibility with IDE 1.6.5 or older, IDE 1.6.6 or newer overrides this value
archive_file_path={build.path}/{archive_file}
recipe.ar.pattern="{compiler.path}{compiler.ar.cmd}" {compiler.ar.flags} {compiler.ar.extra_flags} "{archive_file_path}" "{object_file}"

so in theory we should only use archive_file_path instead of {build.path}/{archive_file}, but this is not true even for our cores.
So a full fix should check for {archive_file} usage in combine recipe and do the right action consequently.

Anyway this is a lot more work for a very edge case.