[RFC] $import directive

[Christopher Collins <ch...@runtime.io>]

Hello all,

I just submitted a newt PR that introduces a new `$import` keyword:
https://github.com/apache/mynewt-newt/pull/260

I thought this feature might benefit from some feedback from the
community, so below is the text from the PR!

All comments appreciated.

Thanks,
Chris

----

PROBLEM:

Within a project, there can be a lot of duplication among target
`syscfg.yml` files.  The Mynewt package hierarchy mitigates this problem
to some extent: the BSP defines some settings, the app overrides a few
of those, and the target overrides yet more.  This does not resolve the
problem fully, however.  Often, a project has several targets that
differ only by one or two syscfg overrides.  For example, a project may
have `dev`, `prod`, and `qa` targets, all nearly identical to each
other.  Duplicating all the target-level overrides in each target is
like any kind of duplication: error prone and a maintenance burden.

PROPOSAL:

A new Mynewt keyword: `$import`.  This allows a Mynewt YAML file to
import the contents of other YAML files.

Example (`targets/dev/syscfg.yml`):

```
$import:
    - 'targets/some_target/syscfg.yml'
    - 'targets/another_target/syscfg.yml'

syscfg.vals:
    OS_DEBUG_MODE: 1
```

In this example, the `dev` target's `syscfg.yml` file imports the syscfg
files of two other targets.  The target inherits all the setting
definitions and values specified in the imported files.

This feature is similar, but not identical, to C's `#include` directive.
Differences between the two features are:

1. Like `#include`, import paths can be absolute or relative.  However,
relative paths are always relative to the project base, not to the file
doing the importing.

2. The `$import` map can be placed anywhere in the file, but its
placement does not affect its behavior.  Imports are always processed
before the rest of the file.

3. Unlike `#include`, `$import` does not perform a simple text
substitution.  If it did, it would result in an invalid YAML file
(multiple `syscfg.defs` or `syscfg.vals` maps).  Instead, maps with the
same name are collected from all imported files and merged into the
importing file.

NOTES

1. This feature introduces the concept of newt keywords.  Keywords are
special directives in a YAML file that alter newt's behavior.  Keywords
always start with "$".  If newt encounters a keyword that it doesn't
understand, it warns the user.

2. This feature may make it confusing to trace down the source of a
syscfg override.  Overrides are still a package-level concept, so the
`target config` and `target [rev]dep` output indicate which package
overrides a setting, not which file.  The implementation of the
`$import` feature attaches file info to each syscfg setting (file that
defines or overrides the setting along with its import tree), but this
information is not accessible at the moment.  I imagine we will want an
alternative `target config` command which displays the file performing
the override rather than the package.