Disable MOZ_B2G_RIL on b2g-desktop completely

[Vicamo Yang]

Hi all,

Bug 920551 [1] tries to allow building B2G phones/tablets without RIL.
The use of MOZ_B2G_RIL and MOZ_WIDGET_GONK is kind of ambiguous now
because we started with RIL features only. In this bug we want to
return the literal definitions for both labels: MOZ_B2G_RIL is for stuff
that only available when RIL functions is available on the target
platform, while MOZ_WIDGET_GONK is for stuff that only available on Gonk.

With recent extended support to Flatfish, code name for Firefox OS
tablet builds, MOZ_B2G_RIL is not necessarily enabled on Firefox OS
platforms even with Gonk.

In reality, MOZ_B2G_RIL is only available when we're building Firefox OS
with Gonk, so MOZ_B2G_RIL can only be enabled when MOZ_WIDGET_GONK is
defined.

Here is the truth-table for the two labels:

| MOZ_WIDGET_GONK |
+----------+------+
| Yes | No |
------------+----------+------+
MOZ_B2G_RIL | Optional | No |
------------+----------+------+


The dom/system/gonk folder should then be guarded by MOZ_WIDGET_GONK,
and same applies to other 'gonk' named backend folders. The files
inside don't need extra MOZ_WIDGET_GONK guard because they're already
guarded. However, they may have a second guard MOZ_B2G_RIL if that
file, or lines of code, is subject to existence of RIL on Gonk.

Outside these gonk folders, FxOS-Gonk specific features, e.g. wifi,
network stats, have to be guarded by MOZ_WIDGET_GONK. Files subject to
RIL have to be guarded by MOZ_B2G_RIL. Files happen to rely on RIL
functionality on Gonk have to be guarded by both MOZ_WIDGET_GONK and
MOZ_B2G_RIL.

!!! IMPORTANT !!!
As a result, you WILL NOT be able to --enable-b2g-ril again on
b2g-desktop. To be clarified, it's NOT enabled by default in our
official b2g-desktop builds.

B2G RIL features on b2g-desktop used to be the debug bridge for physical
Android phones. However, as time goes by, we gradually depend more and
more features that are only available when running with Gonk. The only
practical use-case for B2G RIL on b2g-desktop I've ever heard is to run
RIL xpcshell test cases, and even this is already working fine on B2G
emulator.

If you have other concerns, please don't hesitate to drop a comment on
the bug.

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=920551 ,
"And Bug said, Let there be a macro in the midst of the Gonk, and let it
divide the RIL from the Gonk."