Enable passing vars to configure_file with flags during build [BUILD-391]

[krisukox]

In Bazel, we can neither concatenate dict select (Bazel ticket):

vars = select({
    "//:config-1": {"var1": "ON"},
    "//conditions:default": {"var1": "OFF"},
}) + select({
    "//:config-2": {"var2": "ON"},
    "//conditions:default": {"var2": "OFF"},
}),

nor use select inside a dict:

vars = {
    "var1": select({
        "//:config-1": "ON",
        "//conditions:default", "OFF"
    }),
    "var2": select({
        "//:config-2": "ON",
        "//conditions:default", "OFF"
    }),
},

That's why to enable passing vars to configure_file, we have to use list instead of dict.
List which is passed to configure_file contains vars in the following format: <key>=<value>. Vars which don't contain equal sign are skipped.

I defined a few macros to use configure_file in a more elegant way:
var - define var
config_var - define var that relies on config_setting
config_flag - define flag along with config_setting

PR tested here:
https://github.com/swift-nav/libpal/pull/275
https://github.com/swift-nav/libswiftnav-private/pull/403
swift-nav/gnss-converters-bazel#8

[jungleraptor]

I think this is fine for just us, but it's asking a lot for the end users.

I've spent quite a bit of time today seeing if there's something simpler or more intuitive.

I got close with trying to wrap configure_file in a macro, and abusing kwargs, but it wouldn't bind the select object to the string when passed to the rule: bazelbuild/bazel#3902

I'm leaning toward not changing the API, and just having a config setting for every permutation that you can select on that returns the entire dict.