Convert Android documentation from regular txt format to Sphinx (RST).
Also add Android index.rst file and reference it in root index.rst, so
that Android documentation is visible.

Test:

    $ make htmldocs
    $ xdg-open doc/output/index.html

Signed-off-by: Sam Protsenko <joe.skb7@gmail.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>

Add optional parameter to 'avb verify' sub-command, so that user is able
to specify which slot to use, in case when user's partitions are
slotted. If that parameter is omitted, the behavior of 'avb verify' will
be the same as before, so user API is content.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Acked-by: Igor Opaniuk <igor.opaniuk@gmail.com>

Rename:
 - doc/{README.avb2 => android/avb2.txt}
 - doc/{README.android-fastboot => android/fastboot.txt}

Add a new file documenting the 'bcb' command:
 - doc/android/bcb.txt

The new directory structure has been reviewed by Simon in
https://patchwork.ozlabs.org/patch/1101107/#2176031 .

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Simon Glass <sjg@chromium.org>