android: Enable building ashmem and binder as modules

android: Enable building ashmem and binder as modules

Bug-Debian: https://bugs.debian.org/901492

We want to enable use of the Android ashmem and binder drivers to
support Anbox, but they should not be built-in as that would waste
resources and increase security attack surface on systems that don't
need them.

- Add a MODULE_LICENSE declaration to ashmem
- Change the Makefiles to build each driver as an object with the
  "_linux" suffix (which is what Anbox expects)
- Change config symbol types to tristate

Update:
In upstream commit 721412ed3d titled "staging: remove ashmem" the ashmem
driver was removed entirely. Secondary commit message:
"The mainline replacement for ashmem is memfd, so remove the legacy
code from drivers/staging/"
Consequently, the ashmem part of this patch has been removed.

Gbp-Pq: Topic debian
Gbp-Pq: Name android-enable-building-ashmem-and-binder-as-modules.patch