android-tools: fix dependency on libselinux

android-tools: fix dependency on libselinux

The fastboot sub-option of android-tools did not properly replicate the
dependency of libselinux, which it is selecting. Due to this, an invalid
configuration could be generated, with fastboot selected while thread
support is not available for example (and therefore libselinux is not
available). This problem was causing the following build failure:

 http://autobuild.buildroot.net/results/21e45cee04fd983c85c6702595ee3f7ed8470931/

This is fixed by replicating the selinux dependencies in the fastboot
sub-option and adding the relevant Config.in comment.

In addition, the main android-tools option had some logic to make sure
at least one of its sub-option is enabled: adbd by default on systems
with MMU, and fastboot by default on systems without MMU (because
fastboot is the only part that builds on noMMU systems).

However, with the new dependencies in the fastboot sub-option, this
logic would have become a lot more complicated. Since fastboot is very
unlikely to be used on noMMU systems, we simply make the whole package
not available on noMMU systems.

Cc: Gary Bisson <gary.bisson@boundarydevices.com>
Cc: Antoine Tenart <antoine.tenart@free-electrons.com>
Cc: Julien Corjon <corjon.j@ecagroup.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>