RFC: Aligning all of ANDROID_NDK_ROOT, ANDROID_NDK, ANDROID_SDK_ROOT, ANDROID_SDK

[Friedrich W. H. Kossebau]

Hi,

I would like to propose to change the ECM Android toolchain file and deprecate 
the var
	ANDROID_NDK
and instead use
	ANDROID_NDK_ROOT
Reasons are:
* consistent naming pattern with the other var ANDROID_SDK_ROOT
* ANDROID_NDK_ROOT seems more common than ANDROID_NDK in projects,
  by what searches showed me (though I could not find any "official" naming,
  but chromium developers also seem to use _ROOT:
  https://chromium.googlesource.com/chromium/chromium/+/master/build/android/
envsetup.sh

See http://api.kde.org/ecm/toolchain/Android.html for the current names with 
the ECM toolchain file.

At the same time I also would like to have all KDE projects decide on the same 
naming pattern. Currently there is a mixture (ECM file and those apps having 
their own deploy scripts), which confuses surely not me:

GCompris:
   env vars:
		both ANDROID_NDK & ANDROID_NDK_ROOT, same value
		ANDROID_SDK_ROOT
	cmake var:
		- (at least I could not find a related var)
		ANDROID_SDK_ROOT

Marble:
	env vars:
		ANDROID_NDK
		ANDROID_SDK
	cmake vars:
		QT_ANDROID_NDK_ROOT
		QT_ANDROID_SDK_ROOT

I think we all agree that for synergy effects we should try to share the same 
ECM Android toolchain :) And by what I see both GCompris and Marble, while 
having their own package scripts, are trying to do that. But we could improve 
with those var names, please :) And better now, before more KDE apps for 
Android turn app (actually I start to work on one, that's why I got into this 
mess when trying to set up a generic env).

Would you agree that ANDROID_NDK_ROOT & ANDROID_SDK_ROOT are best?

Then Qt Creator seems to have QT_ANDROID_SDK_ROOT, QT_ANDROID_NDK_ROOT in use, 
oh well.

Opinions, please!

Cheers
Friedrich