[build] Include zip basename directory in build artifacts

[jonpryor]

Commit 1eebbe3 introduces generation of oss-xamarin-android*.zip
files when using the make jenkins target.

There's one "problem" with this file: a common convention for .zip
files is that they will contain a single directory named as the
basename of the .zip file itself:

$ unzip -l foo-1.zip
... foo-1/
... foo-1/file1
... foo-1/file2
...

The benefit to this scheme is that if you have multiple "versions" of
the file, you can extract them without fear of file conflicts:

# extracts into foo-1
$ unzip foo-1.zip

# extracts into foo-2
$ unzip foo-2.zip

The oss-xamarin-android*.zip packages didn't follow this scheme:
instead, they contained the bin directory as it's top directory:

$ unzip -l oss-xamarin.android_v7.0.99.2_Darwin-x86_64_HEAD_fbfd676.zip  | head -10
Archive:  /Users/jon/Downloads/oss-xamarin.android_v7.0.99.2_Darwin-x86_64_HEAD_fbfd676.zip
  Length     Date   Time    Name
 --------    ----   ----    ----
        0  08-10-16 21:46   bin/
        0  08-10-16 23:40   bin/BuildDebug/
    43008  08-10-16 20:10   bin/BuildDebug/api-merge.exe

This means you can't download and extract both
oss-xamarin.android_v7.0.99.1*.zip and
oss-xamarin.android_v7.0.99.2*.zip into the same directory -- not
without extra work -- which is counter to convention.

Fix the make package-oss target so that the zip's basename is
included in the directory structure:

$ unzip -l oss-xamarin.android_v7.0.99.2_Darwin-x86_64_HEAD_fbfd676.zip  | head -10
Archive:  /Users/jon/Downloads/oss-xamarin.android_v7.0.99.2_Darwin-x86_64_HEAD_fbfd676.zip
  Length     Date   Time    Name
 --------    ----   ----    ----
        0  08-10-16 21:46   oss-xamarin.android_v7.0.99.2_Darwin-x86_64_HEAD_fbfd676/bin/
        0  08-10-16 23:40   oss-xamarin.android_v7.0.99.2_Darwin-x86_64_HEAD_fbfd676/bin/BuildDebug/
    43008  08-10-16 20:10   oss-xamarin.android_v7.0.99.2_Darwin-x86_64_HEAD_fbfd676/bin/BuildDebug/api-merge.exe