build: Add "basebranch" to .gitreview

build: Add "basebranch" to .gitreview

If you have a development branch for a major project that
will receive gerrit reviews it'll probably be named something
like "development/16/newproject".  That will necessitate setting
"defaultbranch=development/16/newproject" in .gitreview.  The
make_version script uses that variable to construct the asterisk
version however, which results in versions like
"GIT-development/16/newproject-ee582a8c7b" which is probably not
what you want.  Worse, since the download_externals script uses
make_version to construct the URL to download the binary codecs
or DPMA.  Since it's expecting a simple numeric version, the
downloads will fail.

To get this to work, a new variable "basebranch" has been added
to .gitreview and make_version has been updated to use that instead
of defaultversion:

.gitreview:
defaultbranch=development/16/myproject
basebranch=16

Now git-review will send the reviews to the proper branch
(development/16/myproject) but the version will still be
constructed using the simple branch number (16).

If "basebranch" is missing from .gitreview, make_version will
fall back to using "defaultbranch".

Change-Id: I2941a3b21e668febeb6cfbc1a7bb51a67726fcc4