CLI --enable_platform_specific_config improvements

[siggisim]

Looks like we added support for --enable_platform_specific_config to the CLI in this PR: cbdb3e5

CLI behavior seems to differ from Bazel in a couple of cases:

1. If you pass --enable_platform_specific_config with no arg - that enabled the functionality in Bazel land, but not in CLI land. I suspect that's due to this logic here:

   buildbuddy/cli/parser/parser.go

   Line 806 in 25a335e

   if enable == "true" || enable == "yes" || enable == "1" || enable == "" {

2. If you enable platform specific config for common

common --enable_platform_specific_config=yes

But only have a config set for build

build:linux --foo
build:macos --bar

And you run a

bazel query //...

You get

ERROR: Config value 'linux' is not defined in any .rc file

Which is because we tack on

bazel query //... --config=linux

Which doesn't quite match Bazel's behavior